A review of the structures of oxide glasses by Raman spectroscopy

Abstract

The family of oxide glasses is very wide and it is continuously developing. The rapid development of advanced and innovative glasses is under progress. Oxide glasses have a variety of applications in articles for daily use as well as in advanced technological fields such as X-ray protection, fibre glasses, optical instruments and lab glassware. Oxide glasses basically consist of network formers, such as borate, silicate, phosphate, borosilicate, borophosphate, and network modifiers such as alkali, alkaline earth and transition metals. In the present review article, Raman spectroscopy results for the structures of borate, silicate, phosphate, borosilicate, borophosphate, aluminosilicate, phosphosilicate, alumino-borosilicate and tellurite glasses are summarized.

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1. Introduction

1.1. Brief history of glass

The exact history of glass is not known but there is evidence confirming that it was used around 3500 BCE in Mesopotamia. The available archaeological evidence indicates the presence of glass articles in coastal north Syria, Mesopotamia and Ancient Egypt. The use of glass in South Asia began in 1730 BCE.¹ Evidence of the existence of glass has also been found in Hastinapur and an archaeological site in Takshashila, ancient India. Glass objects have been developed in the Romanian Empire and during the Anglo-Saxon period in domestic, industrial and funerary contexts. In China, the development of glass began late but nowadays, glass plays a peripheral role in arts and crafts. The glasses from that period were made of barium oxide (BaO) and lead.² The tradition of the use of lead–barium glasses disappeared at the end of the Han Dynasty (AD 220). Soda glasses with high alumina content were rarely used in the Mediterranean area or in the Middle East. Soda-lime, borosilicate, glass fibre, lead, alkali-barium silicate, aluminosilicate, vitreous silica and pyrex glasses were investigated from time to time. The large-scale development of glass technology began mainly in the 14^th century.³ Recent research on glasses is based on advanced glasses used for safety systems such as X-ray protective glasses, fibre glasses, high-refractive-index glasses for optical instruments, photo-chromatic and high strength glasses.

In the last few decades, oxide glasses have attracted the attention of researchers and scientists due to their excellent properties, which are very useful in many applications. They are easily shaped due to their lack of underlying crystal structure. Glass covers on solar units and photovoltaic units improve their performance. In turbines, glass fiber-reinforced composites are used for the storage of wind energy. Therefore, boron-free glasses are commonly used.⁴ Oxide glasses may be used in smaller power supplies, including dielectrics for super-capacitors, sealants for high-temperature solid oxide fuel cells (SOFC) and electrolytes for electrochemical devices.⁵ W. H. Zachariasen investigated silicate glasses and he found that such glasses consist of silicon tetrahedrally coordinated to 4 oxygen atoms.⁶ Phosphate glasses were investigated by P. B. Price et al.⁷ Borate glasses have been investigated in various glass systems: B₂O₃–xLi₂O₃,⁸ xB₂O₃–(1 − x)Li₂O₃,^9,10 (30 − x)Li₂O–xK₂O–10CdO/ZnO–59B₂O₃,¹¹ 60B₂O₃–(20 − x)Na₂O–10PbO–10Al₂O₃:xTiO₂:yNd₂O₃,¹² PbO–Bi₂O₃–B₂O₃,¹³ Bi₂O₃–B₂O₃–ZnO–Li₂O,¹⁴ and Gd₂O₃–MoO₃–B₂O₃.¹⁵ Silicate glasses were investigated in the presence of Na₂O, Li₂O, K₂O and CaO by various research groups.^16–25 Phosphate glasses were investigated over a long period of time in many systems. Calcium, sodium, potassium, iron, gadolinium, zinc, lead, chromium, molybdenum, lanthanum, bismuth, tungsten, samarium, copper, cadmium, barium, and silver containing phosphate glasses have been investigated for various applications.^122–165 Soon after the investigation of single-network former glasses, double network former glasses were also investigated for the composite network structures of borate, silicate and phosphate, which are named as borosilicate and borophosphate glasses. In recent years, the structures of SnF₂–SnO–P₂O₅ glasses have been studied. In this study, the glass transition temperature was found to be very low.²⁶ CeO₂ in cerium iron borophosphate glasses increases the glass transition temperature and also increases the thermal stability.²⁷ The thermal stability of Na₂O–FeO–Fe₂O₃–P₂O₅ was found to be poorer than sodium-free iron phosphate glasses.²⁸ TiO₂ is useful for increasing the thermal stability of BaO–Li₂O–diborate glasses.²⁹ Low melting glasses were prepared in the glass system K₂O–MgO–Al₂O₃–SiO₂.³⁰ For the purpose of the immobilisation of nuclear waste, a few borosilicate glasses were also investigated in the glass system 55SiO₂–15B₂O₃–5Al₂O₃–5CaO–(20 − x)Na₂O–xCs₂O.³¹

Glasses may be defined as inorganic solids, which are amorphous in nature. They are classified in several groups such as metallic alloys, ionic melts, aqueous solutions, molecular liquids and polymers. They have attracted the attention of researchers and scientists in recent years due to the demand for glasses in many significant applications in the fields of energy science and photonics. There is still a demand for substantial development of glasses.

1.2. Brief chemistry and physics of glasses

Glasses are disordered materials due to the lack of periodicity in their crystal structures.³² They are formed by the cooling of melted inorganic products to a rigid condition without crystallization.^33,34 Several materials, such as organic polymers and metal alloys, also have amorphous structures but not all of them are glasses.³⁵ The formation of glass requires critical cooling rates.^36–39 Thermodynamically, glasses are non-equilibrium materials because their properties are a function of pressure, temperature and composition. They always approach a nearby metastable state.^40–43 Glass formation is possible if the glass contains network formers. Glasses are formed by network formers such as borate, silicate, phosphate, borosilicate and borophosphate. It has been found that the best glass formers have electro-negativity values of 1.7–2.1 on the Pauling scale.⁴⁴ Network modifiers are used in glass formation to modify the glass properties. The glass network is modified by alkali and alkaline earth metal modifiers such as ZnO, PbO, TeO₂, Bi₂O_3, MgO, CaO, SrO, BaO, CeO₂, Cr₂O₃, La₂O₃, CdO, CuO, Li₂O, Na₂O, K₂O, Al₂O₃ and MgF₂. Alkali metal ions are easily thermally activated and can move from one site to another within a glass. This type of movement of alkali metal ions within a glass structure enables the replacement of alkali metal ions near the surface of a glass by other ions of the same valence.^45,46 Several local packing arrangements are available in glasses, which enable the tuning of their properties through composition and processing. This permits the creation of highly homogeneous materials on a macroscopic scale.⁴⁷ Zachariasen established a theory that explains the criteria for vitreous structures with highly directed three-dimensional bond arrays. Glass structures comprise microcrystallites of ∼20 Å in size. Glass structures lie between a melt state and a glassy state.^48,49 The glassy network structure may be effectively understood by infrared and Raman spectroscopy. Certain viscosity models are used to explain the glass properties on a temperature scale. The five temperatures are named as standard points (associated with the viscous flow of glass), strain point (maximum temperature supported by glass), annealing point (relieves the internal stresses), softening point and working point (most glass forming operations acting at this temperature).⁵⁰

1.2.1 Classification of the preparation method of glasses. Glasses are mainly synthesized by the melt-quench method, sol–gel method and chemical vapour deposition.⁵¹
1.2.1.1 Glass by melt-quench technique. The melt-quench technique is the first discovered technique for the preparation of glasses. In the melt quench method, the inorganic raw materials are accurately weighed according to composition. Then, the raw materials are mixed in a mortar with a pestle or ball mill in acetone media. The well-mixed and dried ingredients are placed into a high-grade alumina or platinum crucible and the crucible is placed in a programmable temperature-controlled furnace for melting. The homogeneous melt is quenched in an aluminium mould. In this way, the glass is formed by the melt quench method. The formed glasses are annealed at relatively low temperatures to the glass casting temperature to remove the residual internal stresses that are produced due to the temperature gradient during the forming and subsequent cooling.⁵² The melt quench method is advantageous for obtaining materials of a large size compared to single crystal or polycrystalline ceramics. It has another advantage as it provides good flexibility of composition over the chemical vapour deposition or the sol–gel method. This method also has certain disadvantages. For example, this method is not suitable for the preparation of glasses with an ultra-high purity that are used in optical communication systems. Certain impurities from the crucible or furnace materials are also added to the glass during this method. Glasses, in which refractory materials such as SiO₂, TiO₂, Al₂O₃ and ZrO₂ are used, are difficult to prepare by this method due to the need for extremely high temperatures.
1.2.1.2 Glass by sol–gel method. In the sol–gel method, a colloidal solution (sol) of raw materials is converted into a gelatinous substance (gel). In this method, there is no requirement of melting and quenching. Fine-grained gel bulks consisting of various organic compounds are annealed at a particular temperature and thereafter the annealed gel is sintered at a higher temperature than the annealing temperature. During this process, there are two reactions taking place, namely, hydrolysis and condensation reactions.⁵³
1.2.1.3 Glass by chemical vapour deposition method. The chemical vapour deposition (CVD) method for the preparation of bulk glasses was developed in the early 1940s. The formation of the glass material is followed by thermally activated homogeneous oxidation or hydrolysis of the initial metal halide vapour. The oxidation or hydrolysis reaction is activated by an oxy-hydrogen flame or oxygen plasma. In this method, the initial components are liquid at room temperature and their boiling temperature is very low compared with alkaline halides, alkaline earth, transition metals or rare-earth elements. The purification of raw materials is achieved by repeating distillation below the melting points. This method is advantageous for the preparation of ultra-high purity glasses.⁵⁴

The structural study of glass networks is important for understanding the composition dependence of oxide glass properties. Various models are proposed for correlating the different properties with the structure and composition of a glassy system. Phillips and Thorpe investigated the compositional dependence of glass properties by analyzing the topology of the glassy network. This approach is based on the comparison of the number of atomic degrees of freedom with the number of interatomic force field constraints. The tendency for glass formation would be maximized if the number of degrees of freedom exactly equalled the number of constraints in the glassy network.⁵⁵ Gupta and Mauro demonstrated the generalization of the Phillips and Thorpe approach in their study.^56,253

This review article is focused on the structural study of various oxide glasses and on how the structure of glassy networks is modified by various dopants such as alkali, alkaline and transition metals. The composition-dependent study of oxide glasses is also summarized in the present review article. This review article is very useful for understanding the formation of glassy networks and their modification by various dopants.

1.3. Brief introduction to glassy networks

A lot of study has been done on glassy materials up to now. Glassy materials consist of various network formers such as borate, silicate, phosphate, borosilicate and borophosphate. The borate network incorporates the vibrations of isolated diborate (B₄O₇²⁻) groups, breathing of O-atoms, loose diborate vibrations, chain or ring type meta- and pentaborate (B₅O₈⁻) groups, symmetric breathing vibrations of six-member borate rings with one or two BO₃ triangles replaced by a (BO₄⁻) tetrahedral, symmetric breathing vibrations of boroxol rings, triborate (B₃O₅⁻), tetra-borate (B₈O₁₃²⁻), ortho-borate (BO₃³⁻), pyro-borate (B₂O₅⁴⁻) groups and linked [BO₃]³⁻ triangles with one free vertex.^45,57 The borate glasses have significant importance in several applications in thin amorphous films for battery application, bioactive glasses for tissue engineering, nuclear waste disposal, photonic applications, development of tuneable or short pulse lasers, optical fibre amplifiers and fibre lasers.^58–61 The borate networks are shown in Fig. 1.

Fig. 1 The network units of borate glasses¹⁰⁷ (reproduced with permission from IOP publishing).

The silicate network former is also significant in glass formation; however silica is not easily fusible and does not retain viscosity for a long time. Thus, it cannot easily form glass. Fluxes are used that help in the formation of glassy networks.⁶² Silicate glasses consist of tetrahedral 6-coordinated 4 oxygen atoms. 6-Coordinated silicon is also found in a few crystalline materials.⁶³ Silicate networks are found in the form of structural units of orthosilicate, Si–O stretching vibrations of tetrahedral silicate units, symmetric stretching vibrations of silicate tetrahedra, inter-tetrahedral Si–O–Si linkages and structural unit Qⁿ_Si (n = 0 to 4), where Q_Si represents the tetrahedral unit and n is the number of bridging oxygens (BO) per tetrahedron.^64–66 Silicate glasses are generally used in windows, lenses, mirror substrates, crucibles, trays and boats, UV-transmitting optics (synthetic fused silica), IR transmitting optics and metrology components. The network units of silicate glasses are shown in Fig. 2. Similar to silicate networks, phosphate networks are formed from various structural units of phosphate. Phosphate glasses are formed at low temperatures compared to silicate glasses. The vibrational units of phosphate networks are Qⁿ_P groups of PO₄ tetrahedra (n = number of bridging oxygens per PO₄ tetrahedron), symmetric stretching of P–O, asymmetric stretching vibrations of P–O, symmetric vibrations of P–O–P, bending vibrations of phosphate polyhedra and O–P–O bending modes. Phosphate glasses have numerous applications in photonics, biomedical applications, solid state electrolytes and fibres.^67–69 Phosphate glasses are very sensitive to ionizing particles. This property of phosphate glasses enables their use in the identification of heavy ions.⁷⁰ The phosphate networks are shown in Fig. 3.

Fig. 2 The network units of silicate glasses¹⁶ (reproduced with permission of the Mineralogical Society of America).

Fig. 3 Network units of phosphate glasses¹⁶⁴ (reproduced with permission from Elsevier).

Not only single network former glasses but also double network former glasses were investigated. Borosilicate glasses are one of the double network former glasses. The co-existence of borate and silicate networks is associated with borosilicate glasses. The interlinking of some borate and silicate networks is also possible in such glasses. The high frequency band of >850 cm⁻¹ is related to silicate networks, Qⁿ_Si, whereas mid frequency bands (400 to 850 cm⁻¹) are associated with ordered superstructures, reedmergnerite [BSi₃O₈]⁻ and danburite [B₂Si₂O₈]²⁻. Bands ranging from 300 to 500 cm⁻¹ are characteristic of mixed stretching and bending modes of Si–O–Si units, whereas bands in the range 550–850 cm⁻¹ are caused by ring breathing modes. The bands at 670, 770 and 808 cm⁻¹ are the signature of tetraborate groups, four- and three-coordinated boron in diborate and boroxol rings, respectively. Borosilicate glasses are mainly used in lab equipment, high quality medical devices (e.g. ampoules, dental cartridges and veterinary tracking devices), space exploration devices and electronics (microelectromechanical systems).^71–74

The properties of borates can be altered by a suitable modification of their chemical compositions within a relatively broad concentration region of their constituents in phosphate glasses. Addition of borate (B₂O₃) to phosphate glasses improves their mechanical properties and chemical resistance to atmospheric moisture. Borophosphate glasses include networks of both borate and phosphate along with various linking vibrations. These glasses are very useful for potential application in solid electrolytes, glass solders and glass seals.^75,76 Phosphosilicate glasses consist of networks of phosphate as well as silicate. Phosphosilicate glasses are used in forming Raman fibre lasers and amplifiers. Phosphosilicate glasses are useful in developing high refractive index gratings.^77,78 Aluminosilicate glasses contain units of silicate and aluminate networks. These are very important in geological applications due to their highly refractory nature.^79,80 Aluminium-based glasses have great technological importance due to their strong and ductile nature. They play an important role where metallic glasses failed.⁸¹ Alumino-borosilicate glasses are formed from networks of aluminate, borate and silicate. These glasses have a wide range of applications in optical, thermal, electrical and mechanical devices. Along with these properties, the addition of Al₂O₃ to borosilicate matrices improves chemical durability, decreases the thermal expansion coefficient, electrical radiation resistant and widens the scope of the applications of borosilicate glasses.⁸² Doping of alkali and alkaline earth metals in these glasses modifies the parent network. Rare-earth ion-doped glasses have importance in applications such as optical fibre lasers, amplifiers and scintillating glasses. Sm²⁺ doped glasses are advantageous in spectral hole burning.^83–88

2. Principles of Raman spectroscopy

Raman spectroscopy is a powerful tool for structural analysis and was developed by Sir C. V. Raman and Kirishnan in 1928.⁸⁹ They pointed out in their study that Raman intensity is 10⁻⁶ to 10⁻⁹ times less than Rayleigh scattering. Such a low intensity can be produced by lasers.⁹⁰ When laser light is incident on vibrating molecules, the energy of photons may be changed. The exited molecules or atoms return to different states. The change in energy between the original state and the new state gives the shift of the emitted photon. The excited molecule goes to a higher or lower frequency than the original state and this phenomenon is known as Stoke Raman scattering, (Stoke shift) or anti-Stoke Raman scattering (anti-Stoke shift), respectively.^91,92 A detailed description of the working principle is shown in Fig. 4. The chemical composition and structure of molecules influence the modified scattering and no two Raman spectra are exactly the same. Thus, Raman shift may be useful in distinguishing the structures of different constituents and molecules.^93,94

Fig. 4 Working principle of Raman spectroscopy.

Raman spectroscopy is used for the determination of the structure, environment and dynamics of glassy materials. Furthermore, the portability of the technique allows its use in on-line process monitoring over other techniques like infrared spectroscopy, NMR and X-ray diffraction.⁹⁵ Raman and infrared (IR) spectroscopy are used as complimentary techniques and both techniques differ slightly according to their selection rules. IR spectra arise from a change in the dipole moment, whereas Raman bands arise from a change in the polarizability. A few transitions are allowed in Raman spectroscopy but are forbidden in IR spectra, so Raman spectroscopy gives particularly useful information in that situation. As with Raman spectroscopy, in FT-IR spectrometry and UV-Vis spectrometry, the characteristics of a sample are analysed using the absorbance (or transmission) spectrum. In IR spectroscopy, molecules like water or acetone are strong IR absorbers, whereas sodium chloride and potassium bromide are very weak absorbers (window materials). Thus, functional groups such as hydroxyl or carbonyl are easily detected, whereas window materials are not easily detected. However, the situation is different for Raman spectroscopy. In Raman spectra, the ordinate axis normally has arbitrary rather than absorption or %T units because it is simply a measure of the number of scattered photons captured by the detector at any particular frequency. If the power of the incident laser on the sample is varied, then the intensity of the Raman spectrum will vary accordingly. Therefore, the peak height in a Raman spectrum is not simply a function of the sample thickness, unlike IR spectra, which depend on sample thickness.

In UV/Visible and IR spectroscopy, the spectral contribution from the instrument is removed by using a reference beam or by subtracting the background spectrum. However, in Raman spectroscopy, any contribution or variation due to the instrument is considered a single-beam. Due to this reason, Raman spectroscopy plays a vital role in quantitative analysis. Unwanted contributions can be minimized by Raman spectroscopy.

X-ray fluorescence (XRF) is also useful for elemental analysis of glassy networks, whereas vibrational networks cannot be significantly understood by XRF. Moreover, a very high energy is required for its operation. X-ray powder diffraction is also used for understanding the nature of glassy materials but this method is particularly useful for the network analysis of glassy materials.^96–98

3. Discussion

3.1. Borate glasses

A typical Raman spectrum of lithium borate glass is shown in Fig. 5.⁹⁹ A Raman study of lithium borate glasses depicts networks of B₍₄₎–O vibrations of tetracoordinated boron at 520.5, 769 and 784 cm⁻¹, vibrations of B₅O₈(OH)₄ units at 555.5 cm⁻¹ and vibrations of tricoordinated boron near 919 cm⁻¹. The characteristic intense peaks at 500.5 and 884.5 cm⁻¹ correspond to the vibrations of B₍₃₎–O in B(OH₃). The sol-derived borate glasses show the presence of boroxol rings as a band found at 807 cm⁻¹.⁹⁹

Fig. 5 Raman spectrum of lithium borate glass⁹⁹ (reproduced with permission from Springer).

Li cations are found in two distinct modes as non-bridging oxygen (NBO) type and bridging oxygen (BO) type sites. The primary network is formed by NBO and BO atoms in tetrahedral borate units, which reside close to NBO atoms. They modify the network by forming clusters around NBO atoms.⁹ The Raman spectra of xLi₂O·(l − x)B₂O₃ glasses consist of one component at 1030 cm⁻¹ corresponding to B–O bond stretching of BO₄ units in diborate polycrystals and others near 900, 940 and 1040 cm⁻¹ corresponding to the same vibration but in tetraborate units. Increasing the concentration of Li₂O leads to the formation of pentaborate, tetraborate, diborate, metaborate and pyroborate.¹⁰ The spectra of Li₂O·2B₂O₃ fluxures show features of a crystalline nature with strong characteristic bands at 783, 1034 and 1174 cm⁻¹ and some weaker bands between 167 and 723 cm⁻¹. Spectra of lithium metaborate (Li₂O·B₂O₃) show clear bands at 545–582, 765, 955, 1108–128 and 1467 cm⁻¹. When the sample is heat treated above the melting temperature, then bands around 640, 671, 724 cm⁻¹ and two less resolved bands in the range 1475–1495 cm⁻¹ were also observed.¹⁰⁰ Raman spectra of Li₂O·4B₂O₃ at room temperature indicate the formation of boroxol rings containing one BO₄ tetrahedron. With increasing temperature, the stability of the BO₃ tetrahedron decreases and it is finally destroyed.¹⁰¹ The networking structure of Li₂O–PbO–B₂O₃ is characterised by [BO_3/2]⁰, [B₂O_4/2]⁻ and the B–O–B bending mode at 1485, 967 and 765 cm⁻¹, respectively. The band due to diborate units at 535 cm⁻¹ is completely eradicated by the addition of PbO, whereas a few additional bands in the region of 280–300 cm⁻¹ are incorporated. The emergence of peaks in the region of 1200 cm⁻¹ is associated with the symmetric stretching of B₃ units and is generated during the incorporation of Pb–O into the structure. The band at 640 cm⁻¹ was attributed to a bending mode of the Pb–O–B links in the new structure.¹⁰² Raman spectra in the range 150–1600 cm⁻¹ of (30 − x)Li₂O − xK₂O − 10CdO/ZnO − 59B₂O₃ (x = 0, 10, 15, 20, and 30) doped with 1 MnO₂/1 Fe₂O₃ showed four regions of bands as follows: (i) 190–600 cm⁻¹; (ii) 600–820 cm⁻¹; (iii) 900–1200 cm⁻¹ and (iv) 1200–1600 cm⁻¹. Strong peaks around 775, 650, 500 cm⁻¹ and a weak peak at ∼805 cm⁻¹ are present, which are attributed to the localized breathing motions of oxygen atoms in the boroxol ring. The peaks around 950 and 1110 cm⁻¹ are due to diborate groups in the structure. The peaks in the high frequency region are attributed to BO₂O⁻ triangles linked to BO₄ units and BO₂O triangles linked to other triangular units.¹⁰³

The polarized Raman spectra of sodium borate glasses showed the short range structure of BO₃ and BO₂O⁻ units. In HH (parallel to the inherent polarisation of the excitation laser) spectra, some strong Raman bands were found in the frequency region 700–850 cm⁻¹. The strongest band due to the boroxol ring was observed at 805 cm⁻¹ in the spectrum of glass. A broader and asymmetric band at the lower frequency side was attributed to the increase of Na₂O. A band near 1500 cm⁻¹ was found in the spectra and this shifts towards the lower frequency side with increasing Na₂O concentration, whereas VH (perpendicular to the inherent polarisation of the excitation laser) spectra showed significant differences compared to HH spectra. VH spectra showed some more bands at 773, 730 and 670 cm⁻¹.¹⁰⁴ The spectra of B₂O₃–Na₂O glasses depict five major peaks at 480, 660, 770, 806 and 920 cm⁻¹. The peaks due to the isolated diborate groups and the ring-type metaborate groups polymerized by BO₃ and BO₄ units remain unaffected by the addition of Al₂O₃, whereas the peak at 770 cm⁻¹ was shifted towards the low frequency side in the spectrum. This meant that BO₄ units were consumed and the number of boroxol rings (B₃O₆)³⁻ gradually increased. Another new vibrational peak at about 920 cm⁻¹ appeared in 6 mol% Al₂O₃. This peak is characteristic of an orthoborate-type structure containing BO₃ units, which can be transformed by the linking of pentaborate and tetraborate groups. The presence of orthoborate groups and the absence of pentaborate groups proves the feasibility of the transformation of pentaborate to orthoborate groups as a result of addition of Al₂O₃ to the structure. This suggests that the addition of Al₂O₃ transformed high polymer borate units like pentaborate into low polymer groups such as boroxol rings, triborate and orthoborate groups.¹⁰⁵ There are four Raman bands in the glassy matrix of 60B₂O₃·(20 − x)Na₂O·10PbO·10Al₂O₃:xTiO₂:yNd₂O₃. The bands at 755 and 772 cm⁻¹ are assigned to the chain-type metaborate groups and the symmetric breathing vibration of six membered rings with one BO₄ tetrahedron, respectively. The position of the boroxol ring was found at 797 cm⁻¹ in this system. When TiO₂ concentration was increased, back conversion of BO₄–BO₃ took place. The incorporation of Ti⁴⁺ led to ∼40% smooth reduction of the BO₄ groups due to the BO₄–BO₃ back conversion effect.¹²

The Raman spectra of lead borate glasses in glass compositions, 80% PbO, 20% Bi₂O₃, 90% PbO, and 1% B₂O₃ showed a very sharp band at 130 cm⁻¹ in the first composition and shifted to 139 cm⁻¹ in the second composition. A few broad bands centred at 710, 905, 1024, 1239 and 1308 cm⁻¹ are also observed in the pattern. In 1 mol% MoO₃ doped lead borate glass, two other peaks at 1927 and 3341 cm⁻¹ are introduced. On increasing the doping concentration of MoO₃, the band near 1255 cm⁻¹ becomes weaker. A sharp band at 133 cm⁻¹ in the 2 mol% Mo-containing lead borate glasses was found. The Raman peak in un-doped lead borate glasses appears at 139 cm⁻¹ and this band was located at 144 cm⁻¹ in crystalline PbO. This suggests the presence of Pb²⁺ in high lead borate glasses in the form of PbO₄ groups. The sharp band observed at 305 cm⁻¹ in lead borate glass, containing 5 mol% of MoO₃, is not observed in un-doped lead borate glasses, whereas it appeared at 296 cm⁻¹ in crystalline lead oxide and 282 cm⁻¹ in crystalline molybdenum trioxide MoO₃. This can be predicted as the Raman band at 305 cm⁻¹ originates from a different site of Pb²⁺. The weak broad Raman bands in MoO₃–lead borate glasses at 3311–3341 cm⁻¹ are assigned to molecular water.¹⁰⁶ Sodium-doped lead borate glasses in the glass system xNa₂O·(100 − x) (10PbO·90B₂O₃) are made from structural units of boroxol rings and growth of triborate.¹⁰⁷ The spectra of bismuth-containing lead borate glasses showed Raman peaks at 400, 550, 710, 920 and 1220 cm⁻¹. The peaks for heavy metal oxides appear in the range 380–580 and 650–950 cm⁻¹ due to the bridging anion modes and the non-bridging anion modes, respectively. The polarization behaviour of bismuth and lead cations is similar due to these having similar atomic weights. The first two peaks correspond to the bridge-anion motion due to symmetric stretching of Bi–O–Bi and Pb–O–Pb combined with Bi–O–Pb.¹⁰⁸ The Raman spectra for xFe₂O₃·(100 − x)[3B₂O₃·0.7PbO·0.3Ag₂O] glasses (Fig. 6) confirm the presence of boroxol rings [B₃O_4.5], pyro-B₂O₅⁴⁻, ditri-[B₃O₈] and dipenta-borate [B₅O₁₁] groups as Raman bands are present at ∼770, ∼800, ∼1040 and ∼1340 cm⁻¹. A wide envelope appears around ∼465 cm⁻¹ due to isolated diborate groups as well as Pb–O linking vibrations and the envelope at ∼700 cm⁻¹ is a characteristic of symmetric breathing vibrations of metaborate rings. The silver ions peak shifted from 806 to 800 cm⁻¹. Increasing the Fe₂O₃ content in lead borate glasses containing silver decreases the intensity of peaks at 770 and 800 cm⁻¹ and increases the intensity of the bands at 1040 and 1340 cm⁻¹.¹⁰⁹ The Raman bands in PbO–BaO–B₂O₃ glasses appear at 770, 806, 1230, 1280 and 1450 cm⁻¹.¹¹⁰ The substitution of cadmium by lead in lead borate glasses was represented by Raman bands in the range of 200–300 cm⁻¹. The band near 840 cm⁻¹ is not found and this suggests that CdO₄ is not formed in the glass network. Raman bands were found at 3691, 1275, 927, 720, 623 and 219 cm⁻¹ in the spectra of glass with composition 30PbO₂·20CdO·50B₂O₃.¹¹¹

Fig. 6 Raman spectra for xFe₂O₃·(100 − x)[3B₂O₃·0.7PbO·0.3Ag₂O] glasses.¹⁰⁹

Barium borate glasses are mainly characterized by two bands near 460 and 805 cm⁻¹. With the addition of 0.5 mol% Mn into the glassy matrix, one more band at about 990 cm⁻¹ appeared and the intensity of this band increases with increasing concentration of Mn, i.e. MnO is helpful for the formation of the orthoborate group. The addition of up to 3 mol% Mn ions causes the intensity of the band at ∼460 cm⁻¹ to increase and thereafter it decreases. Two new bands at ∼630 and 700 cm⁻¹ are also introduced with the addition. The band at ∼630 cm⁻¹ was assigned to the vibration of the ring and chain types of meta- and penta-borate groups, whereas the band at 700 cm⁻¹ is attributed to vibrations of chain or ring-type metaborate groups.¹¹² There are five Raman bands at ∼490, ∼690, ∼800, ∼875 and ∼1250 cm⁻¹ that are present in the 3B₂O₃–As₂O₃ glass matrix. The band at ∼490 cm⁻¹ is caused by the vibrations of isolated diborate groups and/or vibrations of As–O bonds. The band near 875 cm⁻¹ decreases progressively with the addition of silver ions. The Raman spectra of xAg₂O·(1 − x)[zB₂O₃·As₂O₃] (z = 1, 2, 3) glasses showed bands at 490, 685, 803, 880, 960 and 1250 cm⁻¹. The addition of manganese ions to an xAg₂O·(1 − x)[2B₂O₃·As₂O₃] vitreous matrix leads to structural modifications and to an increase in the degree of disorder, particularly for higher manganese ions concentrations (y ≥ 20 mol%) in the glass system x[(1 − y)Ag₂O·yMnO]·(100 − x)[2B₂O₃·As₂O₃].^113,114

Raman studies of xSrO·(1 − x)B₂O₃ (0.2 ≤ x ≤ 0.7) indicate the absence of a band near 806 cm⁻¹, whereas bands are present at 989, 677 and 552 cm⁻¹ for x = 0.2. The Raman spectrum of 0.3SrO:0.7B₂O₃ is dominated by the band at 671 cm⁻¹, which is assigned to di-pentaborate groups, whereas for 0.4SrO·0.6B₂O₃, a band at 799 cm⁻¹ is observed. In a glass sample with composition 0.5SrO·0.5B₂O₃, the band near 744 cm⁻¹ is attributed to six-membered rings with one BO₄ tetrahedron. The intensity of this band decreases with the addition of SrO. Addition of metal cations to these glasses introduced a new band near 423 cm⁻¹. The peak near 1413 cm⁻¹ in the undoped sample was shifted towards a higher wavenumber with the addition of Fe²⁺, Mn²⁺ and Zn²⁺ ions.¹¹⁵ The structure of glasses in the glass system 20MO·55Bi₂O₃·25B₂O₃ (M = Sr, Ba) was determined by FT-Raman spectroscopy. A broad Raman band at 124 cm⁻¹ was observed in both barium bismuth borate and strontium bismuth borate glasses. A few resolved Raman bands are also present in the region 50–400 cm⁻¹. These vibrations are assigned to Bi atoms in crystalline Bi₂₄B₂O₃₉. The external vibration due to α-Bi₂O₃ appeared in the region 0–150 cm⁻¹, whereas the internal vibrations are in the range 150–500 cm⁻¹. The peak centred at ∼128 cm⁻¹ in the Raman spectrum indicates the presence of [BiO₃] and [BiO₆] units in the structure of glass and glass ceramics. Raman bands at 696 and 723 cm⁻¹ are associated with the symmetric bending vibration of the [BO₃]³⁻ anion in Bi₂₄B₂O₃₉ and indicate the violation of the planarity of the [BO₃]³⁻ anion. The band due to the boroxol ring at 804 cm⁻¹ disappeared during heat treatment of glasses. The band at 773 cm⁻¹ is predominant with associated bands at 960, 663 and 487 cm⁻¹. These bands are comparatively weak in barium bismuth borate glass ceramics compared to those for strontium bismuth borate glass ceramics. The overtones of the [BO₃]³⁻ anion are observed in the range of 1100–1500 cm⁻¹. The strong band centred at ∼1412 cm⁻¹ was assigned to linked [BO₃]³⁻ triangles with one free vertex as in BiB₃O₆ heat treated samples. The heat treatment of glasses increases diborate units as indicated by the increased intensity of the bands at 1117, 1031 and 930 cm⁻¹ (strontium containing) and 1107, 1026 and 925 cm⁻¹ (barium containing).¹¹⁶ The Raman study of Bi₂O₃–B₂O₃–ZnO–Li–O showed that Bi can form [BiO₃] pyramidal or [BiO₆] octahedral units. The symmetric stretching anion motion in an angularly constrained Bi–O–Bi configuration was found in the region 300–600 cm⁻¹. The vibrations of Bi–O–Bi and [BiO₆] octahedral units were present near 390 cm⁻¹. The position of this band is shifted towards a lower wavenumber with increasing Li₂O. The shoulder around 555 cm⁻¹ was assigned to stretching vibrations of Bi–O–/Bi–O–Zn and its intensity increases with increasing ZnO content. The Raman peak at 220 cm⁻¹ indicates the presence of Zn–O tetrahedral bending vibrations of ZnO₄ units in the present glass system.¹⁴

The polarized Raman spectra of zinc borate glasses showed features of binary metaborate glasses. The polarized bands appeared at 950, 840 and 1280 cm⁻¹. The two weak bands at ∼770 and 800 cm⁻¹ were also an observed Raman pattern. The unpolarized bands were found at 690 and 1420 cm⁻¹. The strongly polarized shoulder localized at about 250 cm⁻¹ is ascribed to bending modes of ZnO₄ units. Doping of Eu³⁺ in zinc borate increases the intensity of the Raman band. The band at 440 cm⁻¹ is assigned to a Eu³⁺–O stretching/BO₃³⁻ vibrational mode.¹¹⁷ The Raman spectra of glasses in the glass system 60B₂O₃–10TeO₂–5TiO₂–24R₂O:1CuO (where R = Li, Na, K) depict the bands of boroxol rings, vibrations of ring/chain type metaborate units and stretching vibrations of B–O⁻ bonds with non-bridging oxygen. The networking modifying behaviour of Na₂O and K₂O is stronger than Li₂O.¹¹⁸

Raman spectra of (1 − x)[3B₂O₃·K₂O]·xTiO₂ glasses consist of bands centred at 420, 475, 600, 670, 770, 800, 850, 930, 1230 and 1450 cm⁻¹. The intensity of the band at 770 cm⁻¹ is higher than that of the band near 800 cm⁻¹. The band at 770 cm⁻¹ is significant up to x = 0.2. At higher concentrations of titanium oxide, the intensity of bands at 420, 475 and 670 cm⁻¹ was found to increase. For x = 0.5, two shoulders appear at 850 and 600 cm⁻¹. Further addition of TiO₂ introduces the appearance of ring type metaborate groups and loose BO₄⁻ tetrahedra. Therefore, the number of non-bridging oxygens increases with increasing titanium concentration and the glass structure becomes more randomized. The addition of K₂O in borate glasses changes the boron coordination number from 3 to 4.¹¹⁹ Calcium oxide modified the structure of borate glasses. The structure of borate glasses consists of boroxol groups, a smaller number of pentaborate groups, diborate groups, chain type metaborate groups and pyroborate groups at a lower concentration of calcium oxide, whereas at a higher concentration of calcium oxide, pentaborate, orthoborate and metaborate groups also appear. With an increasing content of calcium oxide, the number of non-bridging oxygen ions increases.¹²⁰ Yttrium oxide in calcium borate glasses further modified the structure.¹²¹ Glasses with compositions of 22.5Gd₂O₃·xWO₃·(47.5 − x)MoO₃·30B₂O₃ (x = 0–40) consist of structural units centred at 344, 840 and 944 cm⁻¹, as shown in Fig. 7. When these glasses were heat treated, a few more sharp Raman bands were also introduced. The Raman peak at ∼994 cm⁻¹ was assigned to symmetric stretching vibrations in (WO₄)²⁻ tetrahedral units in the glass composition with Sm₂O₃ for x = 10.¹⁵ Peak assignments of various vibrational units in borate glasses are listed in Table 1.

Fig. 7 Raman spectra of glasses in system 22.5Gd₂O₃·xWO₃·(47.5 − x)MoO₃·30B₂O₃ (x = 0–40)¹⁵ (reproduced with permission from Elsevier).

Table 1 Assignment of main Raman bands in the spectra of borate glasses

Wavenumber (cm^−1)	Raman assignments	Reference
465–500	Isolated diborate groups	105 and 113–115
535	Diborate units	102
600–650	Symmetric breathing vibrations of metaborate rings	100,103,111,115 and 119
650–660	Pentaborate groups	103 and 105
700–735	Symmetric breathing vibrations of metaborate chains	10,12,100,104,106,108,111 and 116
740–775	Symmetric breathing vibrations of six-membered rings with one BO4^− tetrahedron unit	10,12,100,104,105,109,115 and 119
765	B–O–B bending mode	102
800–808	Boroxol ring	12,99,103,105,109,112,112–115,117 and 119
835–840	Pyroborate vibrations	115 and 126
875–1000	Ortho-borate groups	10,105,106,108,111–114,117 and 119
1000–1110	Diborate groups	10,100,102,103,106 and 112–115
1200	Symmetric stretching of B3 units	102
1216–1260	Pyro-borate groups	102,106,108,109 and 119
1300–1600	B–O^− stretching in metaborate rings and chains	102,106,109,115 and 116
3300–3500	Molecular water	102
0–150	External vibration of α-Bi2O3	116
128	Presence of [BiO3] and [BiO6] units	116
150–500	Internal vibrations of α-Bi2O3	116
220	Zn–O tetrahedral bending vibrations of ZnO4 units	117
250	Bending modes of ZnO4 units	118
390	[BiO6] octahedral units	117
300–600	Angularly constrained Bi–O–Bi configuration	117
555	Stretching vibrations of Bi–O–/Bi–O–Zn	117
440	Eu^3+–O stretching/BO3^3− vibrational mode	118

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3.2. Silicate glasses

The networks of silicate glass are shown in Fig. 8. Raman spectroscopy studies of silicate glasses showed the formation of structural units of orthosilicate, silicon–oxygen stretching vibrations of tetrahedral silicate units, symmetric stretching vibrations of silicate tetrahedra, inter-tetrahedral Si–O–Si linkages and the structural unit Qⁿ_Si, where Q represents the tetrahedral unit and n the number of bridging oxygens (BO) per tetrahedron. For silicon compounds, n varies between 0 and 4, where Si is a central tetrahedral atom ranging from Q⁰, which represents orthosilicates SiO₄⁴⁻, Q⁴_Si (tectosilicates), Q³_Si, Q²_Si and Q¹_Si representing intermediate silicate structures.

Fig. 8 The selected networks of silicate glasses¹⁶ (reproduced with permission of the Mineralogical Society of America).

These networks can be modified by adding certain network modifiers such as alkali and alkali earth atoms. The high frequency bands are described in mainly polarized Raman bands centred in the ranges 1050–1100, 950–1000, 900 and 850 cm⁻¹; they are designated as disilicate, metasilicate, pyrosilicate and orthosilicate, respectively. The low frequency bands are mainly associated with Si–O–Si linkages.¹⁶ The presence of the water group in SiO₂ glasses was confirmed by a Raman band at 3598 cm⁻¹. The weak band near 2350 cm⁻¹ arose due to Si–OH groups involved in intratetrahedral hydrogen bonding across an edge of the SiO₄ tetrahedron. The Si–OH stretching mode at 970 cm⁻¹ is more prominent than other bands and shows that more silanol groups are present in comparison to Suprasil. The bands at 430, 800, 1060 and 1200 cm⁻¹ arise due to fundamental vibrations of the dry SiO₂ glass. The sharp peaks at 490 and 600 cm⁻¹ are characteristic of defect bands. Raman bands at 541 and 1100 cm⁻¹ in the spectra of dry and wet sodium silicate glasses indicate the presence of SiO units (Q_Si, species: SiO₄ units with one non-bridging oxygen) and vibrations of the bridging oxygen in the Si–O–Si linkage, respectively. The shoulder peaks near 850, 960 and 1000 cm⁻¹ suggest that three components are present in the high-frequency envelope, whereas four components near 970, 1060, 1100, and 1150 cm⁻¹ are similar to hydrous sodium silicate glasses.¹⁷

The effect of cations on the symmetric vibrational wavenumber of NBO of Si–O–T in the high frequency range is small, whereas the intensity of the Raman peak increases as the radius of the cation increases. The scattering cross section increases in the order of Li, Na, K, Rb and Cs.¹⁸ The Raman spectrum of sulphur-induced sodium calcium silicate glasses indicates that the band due to sulphur ions is located at 990 cm⁻¹. Not only this band, but some other bands due to silicate networks are also present in the pattern.¹⁹ The presence of rare earth oxides in soda-lime-silicate glasses led to a shift of the peak positions at 1100, 790 and 550 cm⁻¹, which are attributed to Si–O–Si asymmetric stretching, Si–O–Si symmetric stretching and bending vibrations, respectively. It was observed that the peak positions of bands at 1100 and 790 cm⁻¹ are shifted towards low frequency and the peak at 550 cm⁻¹ was shifted towards high frequency with doping of lanthanide elements (La₂O₃, CeO₂, Nd₂O₃ and Gd₂O₃) into soda-lime-silicate glass. Y₂O₃ in soda-lime-silicate glass led the frequencies of peaks at 1100, 790 and 550 cm⁻¹ to shift towards the high frequency side. The Qⁿ_Si species in soda-lime-silicate glasses changes as Si₂O₇⁶⁻ + 2Si₂O₅²⁻ = 5SiO₃²⁻ + SiO₂ with addition of lanthanide elements.²⁰ The band near 980 cm⁻¹ was observed in both the bulk (Suprasil) and sol–gel silica glass. The defect mode bands at 493 and 606 cm⁻¹ are also present in the matrix. Raman bands at ∼356, ∼264, ∼207 and ∼128 cm⁻¹ in quartz crystal are attributed to lattice modes, whereas the strong peak at ∼465 cm⁻¹ is characteristic of a symmetric stretching vibration. At higher frequencies, the bands at ∼807 and ∼1083 cm⁻¹ are assigned to Si–O–Si bending and SiO₄ asymmetric stretching vibrations, respectively. The addition of CaO to silica glass changes the relative intensities of bands, as indicated in Fig. 9. The intensity of the band at 1050 cm⁻¹ increased due to addition of CaO. With the addition of a modifier containing MgO, the peak position of 805 cm⁻¹ shifted towards a lower wavenumber. Comparatively, CaO changes the structure more significantly than MgO.^21,22

Fig. 9 Raman spectra of xCaO·(1 − x)SiO₂ (x = 0, 0.1, 0.2, 0.4) glasses²¹ (reproduced with permission from Springer).

The main silicate structure does not change much for smaller contents of metal oxide, but two additional peaks appear at 500 and 600 cm⁻¹. The modifying nature of Cs, Rb, K, Na and Li in silica glass is in the increasing order because the intensity of the peak at 950 cm⁻¹ increases in the same order. The silicate bands broaden in the order Cs < Rb < K < Na < Li < Ca < Mg, which may suggest that the perturbation of silicate units increases with increasing cation strength.²³

Raman spectra of potash-lime-silica glasses depict bands near 300 and 450 cm⁻¹. The addition of sulphur to a glass matrix increases the Raman cross-section of vibrational modes due to S–O bands in comparison to less polarized Si–O bonds. The intense Raman features in such glasses at 1000–1030 cm⁻¹ indicates a mixture of three calcium sulphates: gypsum (CaSO₄·2H₂O), (1008 cm⁻¹), basanite (CaSO₄·1/2H₂O), (1016 cm⁻¹) and anhydrite (CaSO₄), (1026 cm⁻¹). The bands in Raman spectra in the ranges of 608–679 cm⁻¹ and 1110–1167 cm⁻¹ are characteristic of silicate networks, whereas the peak in the range 412–494 cm⁻¹ is characteristic of cation-oxygen vibrational modes.²⁴ In Raman spectra of lead silicate glasses, a band near 1000 cm⁻¹ was shifted towards the longer wavelength and becomes broader with increasing content of lead. The intensity of the Raman peak at 140 cm⁻¹ was increased by increasing the content of lead. Other peaks at 450, 950 and 1060 cm⁻¹ are also modified by PbO. After UV irradiation with an energy density of 150 mJ cm⁻², significant changes in the spectra were observed. The intensity of a Pb–O band at 140 cm⁻¹ decreased after UV irradiation and no new band appeared in the Raman spectra. The decrease in intensity after UV irradiation is caused by the broken Pb–O bond in lead silicate glasses and the broken Pb–O bond is related to the energy density of the UV beam.²⁵ Lead silicate glasses provide a simple absorption spectrum with gamma irradiation.¹²² Band assignments of various vibrational units in silicate glasses are listed in Table 2.

Table 2 Assignment of main Raman bands in the spectra of silicate glasses

Wavenumber (cm^−1)	Raman assignments	Reference
580	Si–O^0 rocking motions in fully polymerized SiO2 (Q^4) units	20
600	Si–O–Si bending vibration in depolymerized structural units	20
700–850	Si–O–Si symmetric stretching of bridging oxygen between tetrahedra	20
1050–1100	Disilicate	16
950–1000	Metasilicate	16
790	Si–O–Si symmetric stretching	20
807	Si–O–Si bending	21 and 22
850	Orthosilicate, SiO4^4−	16
900	Pyrosilicate	16
970	Si–OH stretching mode	17
1083	SiO4 asymmetric stretching vibration	21 and 22
1100	Si–O–Si asymmetric stretching	20
2350	Si–OH groups involved in intratetrahedral hydrogen bonding across an edge of the SiO4 tetrahedron	17

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3.3. Phosphate glasses

The typical Raman spectra of phosphate glass are shown in Fig. 10. The structure of phosphate glasses mainly consists of Qⁿ_P groups of PO₄ tetrahedra (n = number of bridging oxygens per PO₄ tetrahedron). The symmetric stretching of P–O is assigned at 1260 cm⁻¹ of an asymmetric profile (620 cm⁻¹), symmetric stretching vibration of P–O (1380 cm⁻¹), the symmetric stretching of a non-bridging oxygen on a Q²_P tetrahedron at 1170 cm⁻¹, P–O–P symmetric vibrations at 690 cm⁻¹, the symmetric stretching of the orthophosphate groups PO₄³⁻ near 960 cm⁻¹. These bands are modified by dopants such as earth and alkaline-earth atoms. The addition of TiO₂ up to 10 mol% causes the depolymerization of the phosphate glass network by systematic conversion of Q²_P structural units into Q¹_P and finally into Q⁰_P structural units. Even though Q²_P to Q¹_P conversion takes place due to the breaking of P–O–P linkages, formation of P–O–Ti and P–O–Al linkages provides cross linking between short P-structural units. Above 10 mol% TiO₂ content, the network is highly depolymerized due to the formation of Q⁰_P structural units and cross-linking becomes poor. The band near 700 cm⁻¹ is assigned to the symmetric stretching of P–O–P linkages in Q²_P and Q¹_P structural units. On addition of TiO₂ to phosphate glass, the intensity of the bands near 1280, 1170 and 700 cm⁻¹ decreased and a few new bands at 1210, 1090, 900, 750 and 625 cm⁻¹ are introduced. The bands at 1210 and 1090 cm⁻¹ are assigned to asymmetric and symmetric stretching vibrations of PO₃ groups (Q¹ structural units), respectively. The coordination of Ti⁴⁺ ions with oxygen atoms is found in Raman spectra and a band near 625 cm⁻¹ is attributed to the vibration of octahedral titanate units TiO₆, as its relative intensity increases with increasing TiO₂. The band centred at 930 cm⁻¹ is associated with a shorter Ti–O bond present in octahedral titanium and is attributed to the titanyl bonds (Ti–O or –Ti–O–Ti–) associated with five-coordinated Ti. The bands at 900 and 525 cm⁻¹ are related to asymmetric stretching of P–O–P bridges and bending vibrations of P–O bonds, respectively.¹²³ The bands in Raman spectra of a 39AlF₃⋅11NaF⋅10LiF·(40 − x)CaF₂·MgF₂·SrF₂·BaF₂·xNaPO₃ system at 515, 570, and 620 cm⁻¹ are assigned to octahedral [ALF₆], five-coordinated [AlF₅] and tetrahedral [AlF₄] structural units, respectively. The intensity of a band at 1330 cm⁻¹ decreases, whereas the intensity of bands at 1070, 860 and 775 cm⁻¹ increases with increasing phosphate content. The low frequency band at 70 cm⁻¹ is a boson band. Raman peaks at 250 and near the range 350–420 cm⁻¹ are associated with symmetric stretching vibrations of fluorine species bonded by the modifying cations.¹²⁴ The bands at 1194, 1294 and 1349 cm⁻¹ are prominent in the Raman spectra of Na₂O–Al₂O₃–P₂O₅. With an increase in the content of Al₂O₃, the intensity of bands near 1200 cm⁻¹ is enhanced, whereas the band at 1161 cm⁻¹ disappears. The band at 676 cm⁻¹ is shifted towards a higher wavenumber due to the formation of an aluminium metaphosphate structure. Introduction of Al₂O₃ causes the band located at 1349 cm⁻¹ to disappear. The weak vibrations due to PO₂ bending and bending motions in chains of O–P–O appear at 300 and 360 cm⁻¹, respectively. With addition of Fe₂O₃ to this glass system, the intensity of the bands at 1257, 1185 and 706 cm⁻¹ is increased, whereas the position of the band at 1349 cm⁻¹ shifted to 1309 cm⁻¹ due to depolymerization. The ferric oxide in sodium phosphate glasses forms a metaphosphate structure connected by P–O–Fe bonds.¹²⁵ The bands in the Raman spectrum of zinc–alumino–sodium–phosphate glasses with added Pr³⁺ and Nd³⁺ are located at 545, 698, 1042 and 1162 cm⁻¹.¹²⁶ Raman spectra of a 55P₂O₅⋅30CaO·(25 − x)Na₂O·xTiO₂ (0 ≤ x ≤ 5) system have bands near 695, 705, 930, 1180, 1280 and 1370 cm⁻¹. The band at 930 cm⁻¹ is attributed to Ti–O and the intensity of the band at 1370 cm⁻¹ increased by the addition of TiO₂ of up to 5 mol%.¹²⁷ Raman spectra of Li₂O–BaO–Al₂O₃–La₂O₃–P₂O₅ glasses showed bands at 1850, 1700, 1180 and 700 cm⁻¹.¹²⁸ The spectra of phosphate glasses containing TiO₂ consist of bands at 340–540, 693–697, 1010–1020, 1160–1173 and 1248–1271 cm⁻¹. The band in the range 1010–1020 cm⁻¹ disappears at a higher content of TiO₂ and a peak at 905 cm⁻¹ is assigned to asymmetric TiO₅.¹²⁹

Fig. 10 Raman spectra of 40Na₂O–10Al₂O₃–xTiO₂–(50 − x)P₂O₅ glasses.¹²³

Raman investigations of a xMoO₃·(100 − x)[P₂O₅·CaO], 0 ≤ x ≤ 50 mol% glass system showed that Raman bands are centred at 316, 392, 536, 693, 1175 and 1274 cm⁻¹. Following addition of MoO₃ at about 0.3 ≤ x ≤ 1 mol%, a new band at 1032 cm⁻¹ is introduced. The intensity of a band at 683 cm⁻¹ decreased at 5 ≤ x ≤ 10 mol% of MoO₃, whereas a peak at 436 shifted towards a higher wavenumber. The band located at 700 cm⁻¹ becomes weaker and the peak at 1270 cm⁻¹ disappears. MoO₃ is a strong modifier and forms a band at 600 cm⁻¹ due to asymmetric vibration of Mo–O–Mo bond. The band at 894 cm⁻¹ is caused by orthomolybdate, (MoO₄)²⁻ units. The band at 963 cm⁻¹ is assigned to the stretching vibrations of partially isolated Mo–O bonds in deformed (MoO₆) units.¹³⁰ The structure of xCaO·(100 − x) (0.4Fe₂O₃·0.6P₂O₅) (x = 0, 10, 20, 30, 40, 50 mol%) glasses is made up of structural units of phosphate and is modified by calcium as well as iron. Their Raman bands are observed at 626, 772, 950, 1071 and 1241 cm⁻¹.¹³¹ Raman spectra of calcium coacervate show peaks at 1168 and 698 cm⁻¹. The Raman spectrum of the croconate ion consists of bands located at 564, 649, 1605 and 1722 cm⁻¹.¹³² In Raman spectra of glasses in the glass system xGa₂O₃·(1 − x)P₂O₅, there are five Raman bands located at 1320, 1210, 715, 620 and 350 cm⁻¹. Depolarization by Na, Ca, Zn, Mg, Al and Be is found at 350 cm⁻¹. The low frequency band is assigned to Ga–O–M bridges. The band near 640 cm⁻¹ is assigned to a gallate system with four-fold coordinated Ga atoms.^133,134

The structure of PbO–MoO₃–P₂O₅ glasses consists of networks of phosphate units and is modified by MoO₃. The incorporation of MoO₃ results in the transformation of octahedral MoO₆ units into tetrahedral MoO₄ units. The band at 930 cm⁻¹ is shifted towards the high frequency side with increasing content of MoO₃. The intense band near 1158 cm⁻¹ is shifted towards lower frequency with increasing content of MoO₃. The bands in the region 850–970 cm⁻¹ are attributed to Mo–O and Mo–O bonds in MoO₆ tetrahedra.¹³⁵ The structure of PbO–Ga₂O₃–P₂O₅ glasses doped with Cr₂O₃ is modified by the formation of tetrahedral CrO₄²⁻ structural units. The phosphate network is modified by Pb, Ga and Cr ions. The band at 398 cm⁻¹ is assigned to Ga–O–P linkages and the peak at 368 cm⁻¹ is attributed to GaO₆ vibrational groups. The band at 270 cm⁻¹ is associated with Cr–O vibrations of CrO₄²⁻ units.¹³⁶ A lead phosphate network can also be modified by Zn and Cu ions. The phosphate network after modification is assigned to positions 690, 747, 904, 1026 and 1155 cm⁻¹. The intensity of a band at 1155 cm⁻¹ decreases with addition of BaO.¹³⁷ A high ZnO content in lead phosphate glasses forms a network of P–O–Zn bonds.¹³⁸ The network of lead–gallium phosphate glasses can be modified by rare earth ions (Eu³⁺, Dy³⁺, Tb³⁺, and Er³⁺). The spectrum contains two shoulders centred at 1050 and 1120 cm⁻¹, corresponding to symmetric stretching vibrations of diphosphates in Q¹ units. The other two shoulders near 900 and 1250 cm⁻¹ are attributed to symmetric stretching vibrations of PO₄ in the Q⁰_P units and asymmetric stretching vibrations of PO₂ in Q²_P units, respectively. The high frequency band at about 1120 cm⁻¹ is related to the phonon energy of the lead phosphate glass and its intensity decreases when PbO is substituted by PbF₂ components.¹³⁹ The bands near 1172, 1096, 1032, 973, 930, 742, 627 and 474 cm⁻¹ are found in glasses in the system PbO–Fe₂O₃–P₂O₅. The bands at 1080 and 966 cm⁻¹ are related to [P₂O₇]²⁻ and terminal [PO₃]²⁻ symmetric groups in Q¹_P pyrophosphates.¹⁴⁰ In₂O₃ in PbO–P₂O₅–As₂O₃ plays an important role in network modification and forms the structural unit of InO₆. The band centred at 200 cm⁻¹ is related to In–O vibrations of InO₆ structural units. The bands near 110 and 250–350 cm⁻¹ are attributed to Pb–O ionic bond vibrations and PbO₄ structural vibrations, respectively. With an increase in the concentration of In₂O₃, the intensity of the bands due to symmetric vibrations of the phosphate groups is increased, whereas the intensity of bands due to torsional vibrations of PO₄ structural units and As₂O₃ structural vibrations is decreased.¹⁴¹ The fraction of orthophosphate Q⁰_P units in lead iron phosphate glasses increases with increasing Cr₂O₃ content. The band at 600 cm⁻¹ is assigned to different P–O, Fe–O and Pb–O stretching and bending modes. An intense peak appears at 1062 cm⁻¹ in Cr-free glass. The intensity of a band at 979 cm⁻¹ is enhanced with Cr addition. Ferric oxide also forms a network of Fe–O stretching vibrations in FeO₆ octahedra in α-Fe₂O₃ and this is indicated by a band near 614 cm⁻¹.^142,143

The Raman spectra of iron phosphate glasses investigated by Milankovic et al. showed that networks of phosphate glasses are modified by the incorporation of MoO₃. Addition of MoO₃ to a glass matrix replaced the stronger P–O–P bonds with weaker Mo–O–P bonds as the bond covalence decreased from P–O to Mo–O and the bands located at 850 and 980 cm⁻¹ became stronger. Iron works as a network modifier for phosphate networks and a band at 400 cm⁻¹ assigned to PO₄ polyhedra was modified by it. The Raman band near 864 cm⁻¹ is associated with the stretching mode of isolated orthomolybdate (MoO₄)²⁻. Due to an increase in NBO and (PO₄)³⁻, some bands show splitting in the spectra. The asymmetric vibration of Mo–O–Mo is assigned at 620 cm⁻¹, whereas the symmetric Mo–O–Mo vibrational mode appears at 520 cm⁻¹.¹⁴⁴ The addition of La₂O₃ in iron phosphate glasses lowers the peak position of a band at 1072 cm⁻¹ and increases the relative area of a band at 1231 cm⁻¹, whereas this decreases for a band at 1072 cm⁻¹. This result is related to the depolymerization of the phosphate network 2Q¹_P = Q²_P + Q⁰_P.¹⁴⁵ The most intense peak in Raman spectra of cesium-doped iron phosphate glasses is at 1085 cm⁻¹ and is assigned to Q¹_P. This band is shifted towards a lower wavenumber with increasing Cs content. The intensity of a band at 555 cm⁻¹ is enhanced due to the disproportion of the pyrophosphate linkage. The intensity of this band increases with increasing cesium loading and indicates the increasing concentration of orthophosphate units. The bands at 270 and 200 cm⁻¹ are related to bending vibrations of P–O–P and Cs–O vibrations, respectively.¹⁴⁶ The P–O–P network is depolymerized with increasing CeO₂ content in glass compositions and results in the conversion of Q¹_P to Q²_P structural units. The addition of CeO₂ in iron phosphate glasses introduced some new bands near 1000, 1150 and 1279 cm⁻¹. The band at 1037 cm⁻¹ is shifted towards a higher wavenumber with the substitution of FeO₃ by CeO₂. The band at 522 cm⁻¹ is related to FeO bonds and a new peak at 460 cm⁻¹ is attributed to cerium oxygen polyhedron vibration of the [CeO₈] unit symmetrical stretching mode at higher contents of CO₂.¹⁴⁷ Depolymerization of the phosphate network, in the form of conversion of Q¹_P to Q⁰_P groups (2Qⁿ_P + Na₂O = 2Qⁿ⁻¹_P), takes place with the addition of Na₂SO₄ and results in the conversion of P–O–P bridging oxygens to P–O–Na⁺ non-bridging oxygens. The band at 1039 cm⁻¹ is shifted to the higher frequency region with the addition of the modifier Na₂O (Na₂SO₄ = Na₂O + SO₃). The intensity of the band at 931 cm⁻¹ is also increased with this addition.¹⁴⁸ The vibrations at 170, 306 and 420 cm⁻¹ arise due to network vibrations and vibrations of Fe–O polyhedral and phosphate network units, respectively. The peak position of 1090 cm⁻¹ is shifted towards a lower wavenumber with heat treatment at the transition temperature of the glass.¹⁴⁹

The MoO₃ plays an important role in the modification of structural units of the glass system ZnF₂–Bi₂O₃–P₂O₅. With addition of MoO₃ to the glass matrix, the structural units of MoO₄ and MoO₆ are formed. The intensity of the band due to asymmetric vibrations of Mo–O–Mo linkages is observed to increase with an increase in dopant concentration with a slight shift in the position towards a lower wavenumber. In the Raman spectra of the abovementioned system, bands due to BiO₆ and BiO₃ pyramidal structural units are assigned at 547 and 230 cm⁻¹, respectively. The modifier Zn takes the interstitial positions in the form of dangling bonds along with non-bridging oxygens as Zn–O–P linkages and fluorine breaks the P–O network.^150,151 The doping of WO₃ in the place of MoO₃ forms a network of bending vibrations of O–W–O with corner shared octahedra. A noticeable decrease in the intensity of W–O stretching was also observed, which is associated with WO₄ tetrahedral units, as shown in Fig. 11.¹⁵² Trivalent samarium doped K–Mg–Al zinc fluorophosphate glasses are formed by networks of phosphate and modified by K, Mg, Al, Zn and F. The stretching vibration of Al–O linkages in AlO₄ structural units was observed.^153,154 The structure of a 2P₂O₅–CaO–0.05ZnO glass matrix contains Raman bands near 345, 460, 903, 959, 1011, 1137, 1168, 1297 and 1400 cm⁻¹. The content of Ag₂O in the glass system modified the network of ring structures. Some depolymerization had also taken place in the glass matrix. Raman spectra of xAg₂O·(100 − x)[2P₂O₅·CaO·0.05ZnO] confirm the modifying nature of Ca, Zn and Ag. With increasing content of Ag₂O, the intensity of a band near 554 cm⁻¹ is increased and the peak position of a band at 690 cm⁻¹ is shifted towards the lower frequency side due to changes in the chain P–O–P bond angles. Silver oxide favours the presence of orthophosphate, pyrophosphate and small metaphosphate chains; this information is revealed by the increase in intensity of the bands at 554, 1100 and 670 cm⁻¹.¹⁵⁵

Fig. 11 Raman spectra of ZnF₂·Bi₂O₃·P₂O₅ glass-ceramics doped with different concentrations of WO₃ (ref. 152) (reproduced with permission from Elsevier).

Raman spectroscopy investigations of TiO₂–P₂O₅–CaO glasses showed that the structure consists of a distorted Ti octahedral linked to a pyrophosphate unit through P–O–Ti bonds at higher contents of TiO₂ and thus depolymerization took place in the P–O–P network. In Raman spectra of TiO₂-free calcium phosphate glasses, the strong bands near 680, 1175 and 1290 cm⁻¹ are assigned to the glassy network, whereas with addition of TiO₂, the existing bands disappeared. New bands at 765 and 920 cm⁻¹ are also introduced with TiO₂ addition.¹⁵⁶ Li₂O further modified the network of TiO₂–P₂O₅–CaO glasses.¹⁵⁷ Raman spectra of (P₂O₅–K₂O–Al₂O₃–CaF₂) glasses depict bands at 335, 543, 625, 728, 1075, 1135 and 1284 cm⁻¹. The band centred at 728 cm⁻¹ is related to vibrations of P–F chains, whereas a less intense band at 625 cm⁻¹ is assigned to Al₂O vibrations. The band at 335 cm⁻¹ is attributed to K₂O.^158,159 CuO and V₂O₅ work as network modifiers for calcium phosphate glasses. At higher concentrations of CuO and V₂O₅, the glass matrix forms a network of V–O–P and Cu–O–P. The band at 1175 cm⁻¹ indicates depolymerization of the phosphate network. The band at 980 cm⁻¹ is related to V–O vibration in VO₅ tetragonal pyramids, and the stretching vibrations of O–V–O in metavanadate chains appear at 970–905 cm⁻¹.¹⁶⁰ Raman spectra of (50 − y)Na₂O·yCuO·10Bi₂O₃·40P₂O₅ (0 ≤ y ≤ 25 mol%) glasses depict the formation of P–O–Bi and P–O–Cu bonds. Bi₂O₃ acts as a network modifier and is incorporated in the form of BiO₆ units. No depolymerization took place with the replacement of Na₂O by CuO.¹⁶¹ As CuO content increases in xCuO·(1 − x)P₂O₅ glasses, the intensity of the P–O band associated with the Q³ Cu tetrahedra increases to x < 0.33 and decreases with a concomitant increase of the intensity of the band at 1265 cm⁻¹ due to the asymmetric vibration of the PO₂ groups on Q² tetrahedra. When x > 0.33, the isolated Cu-octahedra begin to share common oxygens to form a sub-network in the phosphate matrix. The intensity of a band at 640 cm⁻¹ increases with the addition of CuO. CuO in phosphate glasses promotes depolymerization.¹⁶²

Raman spectra of cadmium phosphate glasses reveal the formation of P–O–V and V–O–V bonds instead of P–O–P vibrations after the addition of V₂O₅. The presence of a band at 760 cm⁻¹ is attributed to V–O–V vibration. The intensity of bands at 615 and 974 cm⁻¹ is increased with addition of V₂O₅ at about 20 mol%. These bands are related to P–O–V and V–O vibration of VO₅ trigonal bipyramids.¹⁶³ The Li⁺ and Cd²⁺ cations in the phosphate glass network are responsible for shifting the bands as well as for intensity modification. The band at 1120 cm⁻¹ is shifted towards the high frequency side with increase of the Li₂O content.¹⁶⁴ Increasing the content of BaO in lithium barium phosphate glasses causes the intensity of all bands to be enhanced. The bands at 712 and 1163 cm⁻¹ are shifted slightly towards a lower wavenumber with an increase in BaO. The decrease in wavenumber is related to an increase in the average length of the P–O bond resulting from π-bond delocalization on the Q³_P tetrahedra.¹⁶⁵ Raman spectra of phosphate glasses with addition of Er³⁺/Yb³⁺ ions depict bands at 312, 380, 685, 1165 and 1275 cm⁻¹.¹⁶⁶ The assignments of the main Raman bands in the spectra of phosphate glasses are listed in Table 3.

Table 3 Assignment of main Raman bands in the spectra of phosphate glasses

Wavenumber (cm^−1)	Raman assignments	Reference
1300–1350	P=O stretching of terminal oxygen	123
1267–1294	PO2 asymmetric stretching of non-bridging oxygen in Q^2 units	123
1260, 1380	Symmetric stretching of P–O	123
1210	Asymmetric stretching vibrations of PO3 groups	123
1170	Symmetric stretching of a non-bridging oxygen on a Q^2P tetrahedron	123
1161	PO2 symmetric stretching of non-bridging oxygen in Q^2 units	123
1065–1097	PO2 symmetric stretching of non-bridging oxygen in Q^1 units	123
1090	Symmetric stretching vibrations of PO3 groups	123
1080	[P2O7]^2− symmetric groups in Q^1P pyrophosphates	140
966	Terminal [PO3]^2− symmetric groups in Q^1P pyrophosphates	140
960	Symmetric stretching of the orthophosphate groups PO4^3−	123
940	PO2 symmetric stretching of non-bridging oxygen in Q^0 units	123
900	Asymmetric stretching of P–O–P bridges	123
700	The symmetric stretching of P–O–P linkages in Q^2P and Q^1P structural units	123
690	P–O–P symmetric vibrations	123
676–708	P–O–P symmetric stretching of bridging oxygen in Q^2 units	123
525	Bending vibrations of P–O bonds	123
390–410	O–P–O bending modes	123
398	Ga–O–P linkages	136
360	Chains of O–P–O bending motions	125
300	PO2 bending O–P–O bending motions	125

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3.4. Borosilicate glasses

Typical Raman spectra of alkali borosilicate glasses are shown in Fig. 12. The network structure of borosilicate glasses is formed by borate as well as silicate structural units. The structure of lithium silicate glasses produces Raman bands at 850 cm⁻¹ (Q²_Si), 950 cm⁻¹ (Q²_Si), 1000 cm⁻¹ (Q³_Si), 1050 cm⁻¹ (Q³_Si), 1080 cm⁻¹ (Q³_Si) and 1150 cm⁻¹ (Q⁴_Si). The borosilicate structure is associated with the vibrational modes of the two medium-range (400 to 850 cm⁻¹) order superstructures, reedmergnerite [BSi₃O₈]⁻ and danburite [B₂Si₂O₈]²⁻ and high frequency region (850–1250 cm⁻¹) vibrations are related to silicon Qⁿ_Si vibrational modes.¹⁶⁷

Fig. 12 Raman spectra of alkali borosilicate glasses¹⁶⁷ (reproduced with permission from Elsevier).

Raman spectra of sodium borosilicate glasses depict bands centred at 350, 500, 630, 670, 770 and 808 cm⁻¹. The bands between 300 and 500 cm⁻¹ are attributed to mixed stretching and bending modes of Si–O–Si units. The bands in the 550–850 cm⁻¹ range are caused by ring breathing modes. The bands at 670, 770 and 808 cm⁻¹ are the signature of tetraborate groups, four- and three-coordinated boron in diborate and boroxol rings, respectively. The intensity of the peak at 808 cm⁻¹ decreases with increasing sodium content, whereas the band near 770 cm⁻¹ shows the opposite behaviour. This means that sodium ions favour the formation of BO₄⁻ units at the expense of BO₃ units. Polymerization of the silicate network first increases with Na₂O content up to Na₂O/B₂O₃ = 0.6 and thereafter decreases with increasing Na₂O content.¹⁶⁸ Sulphate in borosilicate glass showed its presence in the form of vibrational modes of SO₄ tetrahedra. The major bands of sulphate at 1100, 1000, 990, 620 and 460 cm⁻¹ are related to asymmetric S–O stretch modes, symmetric S–O stretching, symmetric S–O stretch vibrations of tetrahedral SO₄²⁻, asymmetric O–S–O bend modes and symmetric O–S–O bend modes, respectively.¹⁶⁹ When the B₂O₃ content is increased, broad bands near 1500 and 800 cm⁻¹ are developed. The shoulder peak at 590 cm⁻¹ is assigned to the symmetric oxygen breathing vibrations of three-membered siloxane rings. The network modifier ions Na⁺ and Ba²⁺ are used for the conversion of BO₃ units into BO₄ units before forming non-bridging oxygen (NBO) in the network. As the silica content increases in the glassy matrix, the ratio Na/Si (or Ba/Si) decreases, which could depolymerise the silicate network after the formation of BO₄ units. It can be concluded that the silicate network in SiO₂-rich borosilicate glasses remains more polymerized than in boron-rich glasses.¹⁷⁰ The bands in the range 1200–1100 cm⁻¹ disappear as the ratio B₂O₃/Na₂O tends to zero. The intensity of the band at 1077 cm⁻¹ is also increased. With an increasing amount of Na₂O in the glass, the intensity of the band around 630 cm⁻¹ increased and the band in the range 550–450 cm⁻¹ began to dominate, due to formation of [BO₄] tetrahedra with a small degree of polymerization. The intensities of bands at 385, 360, 287, 1850 and 1975 cm⁻¹ are increased as the ratio B₂O₃/(Na₂O + 3La₂F₆) increases.¹⁷¹ When barium is substituted for the sodium cation, the position of the peak at 540 cm⁻¹ is shifted to the low frequency region and the intensity of a band at 635 cm⁻¹ was decreased. A high frequency band around 1100 cm⁻¹ becomes stronger with the introduction of sodium. The substitution of Ba and Ca cations enhances the intensity of a band at 960 cm⁻¹ and shifts it towards the low frequency region.¹⁷² Raman spectra of Na₂O–B₂O₃–SiO₂ exhibit bands at ∼430, 485, 600, 800, 1060 and 1200 cm⁻¹. Bands around 485 and 600 cm⁻¹ (known as defect lines D₁ and D₂) are assigned to the ring breathing mode of four and three-membered silicate rings, respectively. Raman peaks at 770 and 805 cm⁻¹ become more significant for glasses with higher borate content.¹⁷³ In Raman spectra of 30Na₂O·5SiO₂·65[(1 − x)P₂O₅·xB₂O₃] glasses, the intensity of bands at 680 and 770 cm⁻¹ decreases with an increase in B₂O₃; these are attributed to (P–O–P)_sym stretching vibrations and BO₄ stretching in borate species, respectively. Bands at 1200 and 1350 cm⁻¹ disappear for x = 0.250. The intensity of a band at 620 cm⁻¹ increases with an increase in the B₂O₃ content. Bands appear at 500, 780 and 1100 cm⁻¹ in B₂O₃ rich glasses. The Raman band at 1160 cm⁻¹ is attributed to the symmetric stretching mode of the terminal oxygen on each tetrahedron. The intensity of a band at 780 cm⁻¹ increases and is assigned to a BO₃ stretching vibration.¹⁷⁴ The intensity of Raman bands at 810 cm⁻¹ is increased, whereas it is decreased for a band at 795 cm⁻¹ as a result of heat treatment of glasses.¹⁷⁵ The S–O symmetrical stretching modes near 1000 cm⁻¹ from tetrahedral SO₄ environments were observed. The peak near 300 cm⁻¹ is assigned to S–S stretching in the dithionate anion S₂O₆²⁻.^176,177

The wettability of diamond by borosilicate glass melt at temperatures above 700 °C was enhanced due to oxidation of the diamonds. The irregular pores become distinct in diamond–borosilicate glass composites sintered above 800 °C.¹⁷⁸ Depolarization of the silicate network takes place with an increase in zirconia and causes reduction of the intensity of the band due to Si–O–Si links. Some of the highly polymerized Q³_Si and Q⁴_Si units are replaced by the less polymerized Q²_Si units in the glass structure. Most of the borate units within the glass structure are probably linked to silicate tetrahedra in the range 650–800 cm⁻¹ and this is indicated by the lack of any prominent peaks in this range. The band near 703 cm⁻¹ is related to the zirconia network (Zr–O stretch).¹⁷⁹ The spectrum of Eu³⁺ ion doped MgO–PbO–B₂O₃–SiO₂–Nd₂O₃ glasses consists of bands centred at 1510, 1385, 955, 842, 732, 645, 487 and 298 cm⁻¹ (Fig. 13). The latter band near 298 cm⁻¹ was attributed to Ln–O–Ln (Nd and Eu) stretching vibrations. The position of the first band was shifted towards a higher wavenumber with an increase of the doping concentration of Eu. The bands at 605 and 527 cm⁻¹ are related to B–O–Si linkages along with PbO₄ units and Si–O–Pb, respectively. Polymerization of the glass network takes place as a result of replacement of B–O–B and Si–O–Si bonds with more resistant B–O–Si and Si–O–Pb bonds with addition of Eu³⁺ ions.¹⁸⁰ Structural units of TiO₄ exist in CaO–B₂O₃–SiO₂–TiO₂ glasses and the degree of depolarization decreases with an increase in the CaO/SiO₂ ratio. An increase in B₂O₃ leads to an increase of BO₄ tetrahedral units in the glass matrix. The addition of TiO₂ to this glass system introduces two peaks near 840 and 726 cm⁻¹, whereas the existing bands at 500–800 cm⁻¹ disappear. Bands near 948, 1040 and 1300–1500 cm⁻¹ shifted towards low frequency. The bands at 840 and 726 cm⁻¹ are characteristic of the vibrations of Ti–O–Si or Ti–O–Ti and deformation of O–Ti–O or O–(Si,Ti)–O in chain or sheet units or both.¹⁸¹

Fig. 13 Raman spectra of MgO–PbO–B₂O₃–SiO₂–Nd₂O₃–Eu₂O₃ glasses¹⁸⁰ (reproduced with permission from Elsevier).

The structure of borosilicate glass in the presence of the modifier BaO also contains a network of borate and silicate. The addition of BaO decreases the intensity of a band near 816 cm⁻¹. A new band at 780 cm⁻¹ is incorporated at 42 mol% of BaO. Bands at ∼843, 877, 934, 1000 and 1040 cm⁻¹ are shifted to the lower frequency region and are caused by low network polymerization after the formation of BO₄ units due to the increase in BaO content instead of SiO₂.¹⁸² The position of a peak at 675 cm⁻¹ is shifted towards a higher wavenumber with an increasing ratio of BaO/SrO. This is associated with variation in the ionic radii of Ba and Sr. La₂O₃ and Fe₂O₃ in barium strontium titanate (BST) borosilicate glasses shift the peak positions of 675 and 820 cm⁻¹ towards a higher wavenumber.^183–185 A Raman band at about 735 cm⁻¹ occurs due to metaborate groups and symmetric breathing vibrations of BO₃ triangles replaced by boron tetrahedra (BO₄). The Raman shift has been shifted towards a higher wavenumber with an increase in the doping concentration of Fe₂O₃.⁵² Raman spectra of AO–SiO₂–B₂O₃–La₂O₃, (A = Mg, Ca, Sr, Ba) glasses showed bands near 770–779 and 1274–1280 cm⁻¹, which are related to borate networks. The bands near 796–800 and 1060–1070 cm⁻¹ are caused by silicate networks. Tetrahedral coordination occurs in both silicon and boron in this glass system. Lanthanum occupies interstitial octahedral co-ordination in the local network. Modifiers, such as Ba, Ca, Mg, Sr and La, break the B–O or Si–O bonds and occupy interstitial or substitutional positions in the glass network. As the ionic radii of the modifier ion increases in the order Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺, polarization follows Fajan's rule, i.e. weak polarization by Mg and strong polarization by Ba.¹⁸⁶ Raman spectra of lead bismuth borosilicate glasses show bands at 1582 and 1382 cm⁻¹ in the high frequency region.¹⁸⁷ After irradiation, the peak at 480 cm⁻¹ is shifted towards higher wavenumber. This suggests a decrease of the Si–O–Si bond angle. A new weak peak at 605 cm⁻¹ is introduced by irradiation in glasses and is assigned to the D₂ defect peak involving the three-membered rings of SiO₄ tetrahedra. The weak band at 630 cm⁻¹ is associated either with the hindered bending vibration of ring-type metaborate groups or the breathing mode of danburite-like rings. The Qⁿ_Si species of silicon remains almost unchanged after Ar-irradiation.¹⁸⁸ The Raman band assignments of borosilicate glasses are listed in Table 4.

Table 4 Assignment of main Raman bands in the spectra of borosilicate glasses

Wavenumber (cm^−1)	Raman assignments	Reference
1510–1570	B–O stretching modes involving one NBO of [BO3] triangles and molecular oxygen stretching vibration modes	183–185
1385–1397	Stretching vibrations of B–O^− bond in BO4 units from different borate groups	183–185
1160	Symmetric stretching mode of the terminal oxygen on each tetrahedron	174
947–955	Stretching vibrations of Si–O bond due to the presence of BO4 units	183–185
840–842	Pyroborate unit along with ortho-silicate (SiO4)^4− composition or vibrations of Ti–O–Si or Ti–O–Ti structural units	181
808	Boroxol rings	168
722–732	Chain-type metaborate groups containing NBO or deformation of O–Ti–O or O–(Si,Ti)–O in chain or sheet units or both	181
550–850	Ring breathing modes	167 and 168
400–850	Reedmergnerite [BSi3O8]^− and danburite [B2Si2O8]^2−	167
605	B–O–Si linkages along with PbO4 units	180
590	Symmetric oxygen breathing vibrations of three-membered siloxane rings	170
300–500	Mixed stretching and bending modes of Si–O–Si units	167
475–487	Si–O–Si, Si–O–B isolated vibrations	180

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3.5. Borophosphate glasses

Typical Raman spectra of tin borophosphate glasses are shown in Fig. 14. In this study, it was found that the Raman bands were centred at 1330, 1270, 1050, 1000, 950, 760, 700 cm⁻¹ and some bands appeared near 500 cm⁻¹. The vibrations at 1330 and 700 cm⁻¹ are attributed to metaborate groups, whereas the band near 1000 cm⁻¹ is attributed to pyrophosphate or orthophosphate units. The presence of metaborate chains in the glass matrix was indicated by the vibration at 760 cm⁻¹. Evidence of pyrophosphate units was also confirmed by the peak located at 1050 cm⁻¹. The vibration at 1270 cm⁻¹ is caused by orthoborate/pyroborate units.¹⁸⁹ A network of lithium borophosphate glasses, in the presence of germanium oxide, consists of stretching vibrations of phosphate structural units in high (1000–1300 cm⁻¹) and mid frequency regions (500–700 cm⁻¹) and these units are modified by Ge ions. Ge-free glasses showed a band at 1159 cm⁻¹, which is caused by a metaphosphate structure. The vibration at 1260 cm⁻¹ is related to an asymmetric, ν_as(PO₂), stretching mode in Q²_P units, whereas the weaker band centred at 1093 cm⁻¹ is attributed to ν_s(PO₃) in Q¹_P units. The bands at 703 and 665 cm⁻¹ are assigned to the symmetric stretching mode of the P–O–P bridging bond in Q¹_P and Q²_P structures, respectively. A broad band around 320 cm⁻¹ is attributed to the cage vibrational frequencies of Li⁺ ions. The addition of 5 mol% GeO₂ shifted the band around 1159 to 1106 cm⁻¹ and also caused depolymerization of the Q²_P structure by the formation of more Q¹_P phosphate units. The intensity of the band at 665 cm⁻¹ is reduced with increasing GeO₂ and it is completely eradicated by the addition of 15 to 25 mol% of GeO₂, whereas the intensity of bands at 700 and 751 cm⁻¹ is enhanced due to more depolymerization of Q¹_P units by this addition. A new band at 600 cm⁻¹ is introduced due to Ge–O–Ge bending modes or Ge–O–P stretching modes.¹⁹⁰ Four intense bands at 650, 696, 880 and 1112 cm⁻¹ are found in the Raman spectrum of 75P₂O₅–20B₂O₃–4.9Na₂O–0.1Er₂O₃ glass.¹⁹¹

Fig. 14 Raman spectra of various tin borophosphate glasses¹⁸⁹ (reproduced with permission from Elsevier).

Raman bands at 636 and 350 cm⁻¹ in Na₂O–Ga₂O₃–P₂O₅ glasses are attributed to GaO₄ and GaO₆ groups, respectively. The bands centred at 515 and 540 cm⁻¹ are caused by Ga–O–Ga bonds between GaO₄ tetrahedra, and the band at 675 cm⁻¹ is associated with vibrations involved with non-bridging oxygens from the GaO₄ tetrahedra. The band at ∼540 cm⁻¹ was attributed to [GaO₄]⁻. In Ti-containing glasses, an intense band near 900 cm⁻¹ is caused by [TiO₄] groups.¹⁹² The Raman spectra of calcium borophosphate consisted of bands attributed to networks of ortho Q⁰_P and pyro Q¹_P phosphate units. With the addition of 20 mol% of B₂O₃ to calcium phosphate glass, a band at 1045 cm⁻¹ is shifted to 1038 cm⁻¹ and new bands are introduced at 1000 and 675 cm⁻¹.¹⁹³ In the Raman spectra of (1 − x)[0.5K₂O·0.1B₂O₃·0.4P₂O₅]·xNb₂O₅ glasses, a strong band in the region 907–911 cm⁻¹ was observed and the bands assigned to phosphate groups appear at 1070–1148 cm⁻¹ and 1212–1244 cm⁻¹. The new bands at 811–817 and 638 cm⁻¹ are introduced in glasses for more than 16.7 mol% Nb₂O₅. The vibrations centred at 907 cm⁻¹ are characteristic of symmetric stretching vibrations of isolated NbO₆ octahedra. The dominating bands at 817 and 638–648 cm⁻¹ are assigned to Nb–O–Nb bonds in glass with 37.5 mol% of Nb₂O₅. The bands at 820–840 cm⁻¹ in barium–potassium niobato–phosphate glasses are attributed to the deformation vibrational modes of Nb–O–Nb bridges between NbO₆ octahedra.¹⁹⁴

Raman spectroscopy studies of 50PbO·10B₂O₃·40P₂O₅·xTiO₂ glasses showed that major bands are located at 329, 729, 936, 1100 and 1210 cm⁻¹ (Fig. 15). The strong peaks at 1100 and 1210 cm⁻¹ are shifted towards a lower wavenumber, whereas the peak at 729 and 936 cm⁻¹ are shifted towards a higher wavenumber with increasing content of TiO₂.¹⁹⁵ TeO₂ in lead borophosphate glasses forms the structural units of TeO₃ and TeO₄ at low and high concentrations, respectively. The intensity of a band at 1092 cm⁻¹ decreases with TeO₂ content and also shifts towards the low frequency side. This change is associated with the depolymerisation of the phosphate network. The bands at 550 and 850 cm⁻¹ are attributed to the vibration of oxygen atoms in P–O–P bridges and are gradually replaced by a band that is a characteristic of different Te–O vibrations. New bands in the 453–458 cm⁻¹ region are ascribed to bending vibrations of O–Te–O and Te–O–Te linkages. The bands lying at 620–630 cm⁻¹ and 660–670 cm⁻¹ are related to the stretching vibrations of TeO₄ trigonal bipyramids, whereas the vibrations at 720–750 cm⁻¹ and 760–800 cm⁻¹ are associated with stretching vibrations of TeO₃ trigonal pyramids.¹⁹⁶

Fig. 15 Raman spectra of the glass series 50PbO–10B₂O₃–40P₂O₅ + xTiO₂ (ref. 196) (reproduced with permission from Elsevier).

Zinc oxide plays an important role in the modification of borate and phosphate structural units. In the spectra, a sharp band at 808 cm⁻¹ is characteristic of B₂O₃ and the band at 968 cm⁻¹ is caused by the vibrations of isolated PO₄ units in the structural network of borophosphate glasses.¹⁹⁷ If TiO₂ is added to zinc borophosphate glasses, the intense bands at 1162 and 666 cm⁻¹ are shifted towards the lower frequency side, whereas the band at 747 cm⁻¹ is shifted to 774 cm⁻¹. The band at 506 cm⁻¹ disappeared above 4 mol% of TiO₂. The shifts of bands are associated with the distortion of borate and phosphate units. The shift in sodium containing glasses is less than that in Zn modified glasses due to the difference in the ionic field strength.¹⁹⁸ All the Raman bands are shifted towards a lower wavenumber with the substitution of Sb₂O₃ instead of TiO₂ in the abovementioned glass system. The vibrations of structural units containing Sb–O are assigned in the range 350–700 cm⁻¹. Sb₂O₃ modifies the network in the form of SbO₃ units with a single atom of Sb. The low concentration of Sb₂O₃ forms the isolated SbO₃ unit. The existence of three bands in place of two is associated with the splitting of the symmetry of SbO₃ units in the glass network. The Sb₂O₃ content replicates the depolymerisation of phosphate chains. Raman bands in the 520–690 cm⁻¹ range are attributed to SbO₃ pyramids. SbO₃ units are linked into chains and clusters with Sb–O–Sb bridges at higher Sb₂O₃ contents and are characterised by bands in the 380–520 cm⁻¹ regions.¹⁹⁹ MoO₃ weakened the bond strength for zinc borophosphate glasses. The presence of the polarized vibrational band at 976 cm⁻¹ is associated with MO_x symmetric stretching vibrations and the depolarized band at 878 cm⁻¹ is ascribed to the Mo–O–Mo stretching vibration. The incorporation of molybdate units in the glass matrix is responsible for the depolymerization of phosphate chains and the formation of P–O–Mo bonds.²⁰⁰ Tellurium oxide in the abovementioned glass system forms structural units of TeO₃, TeO₃₊₁ and TeO₄. The phosphate units are depolymerized by the incorporation of TeO₂ and also boron coordination is modified. When the ratio of B₂O₃/P₂O₅ increases, TeO₄ units are replaced by TeO₃ units as the number of oxygen atoms in the glass decreases.^201–204 The peak assignments of Raman spectra of borophosphate glasses are listed in Table 5.

Table 5 Assignment of Raman bands in the spectra of borophosphate glasses

Wavenumber (cm^−1)	Raman assignments	Reference
1260, 1380	Symmetric stretching of P–O	123
1210	Asymmetric stretching vibrations of PO3 groups	123
1170	Symmetric stretching of a non-bridging oxygen on a Q^2P tetrahedron	123
1080	[P2O7]^2− symmetric groups in Q^1P pyrophosphates	140
966	Terminal [PO3]^2− symmetric groups in Q^1P pyrophosphates	140
960	Symmetric stretching of the orthophosphate groups PO4^3−	123
600	Ge–O–Ge bending modes or Ge–O–P stretching modes	190
550, 850	Vibration of oxygen atoms in P–O–P bridges	196
968	Vibrations of isolated PO4 units	199

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3.6. Aluminosilicate glasses

Raman spectra of glasses for the glass system 5Na₂O·20CaO·5Al₂O₃·(60 − x)SiO₂·xZnO (where x = 0, 4, 7 and 10 mol%) showed the bands by deconvolution, which are centred at 868, 956, 1020 and 1075 cm⁻¹ and are attributed to the Si\NBO stretching modes of mainly silicate units, Qⁿ_Si (Fig. 16). The band near 868 cm⁻¹ is characteristic of Q¹_Si. The peak at 956 cm⁻¹ is assigned to Q²_Si, whereas the peak at 1019 cm⁻¹ is caused by the asymmetric stretching of BO and is related to Q¹_Si–Q³_Si units. The band at 1020 cm⁻¹ is associated with Q³_Si. Raman bands in the wavenumber range of 400–800 cm⁻¹ are at 412, 544, 622 and 779 cm⁻¹. The vibrations in the range 400–650 cm⁻¹ are caused by the bending vibrations of the bridging oxygen (BO) bonds of SiO₄ and the band near 779 cm⁻¹ is caused by the Si–O–Si network, ZnO₄ tetrahedra and AlO₄ units with three BOs and one NBO. The low frequency bands located at 222, 279 and 338 cm⁻¹ are related to the cations of modifying networks. The vibration of cations Na⁺ and Ca²⁺ are centred at 222 and 279 cm⁻¹, respectively. The coupled vibration of the Ca–O and SiO₄ networks is found at 338 cm⁻¹. The addition of ZnO to the glass matrix shifts the bands at 279 and 338 cm⁻¹ to 274 and 346 cm⁻¹, respectively. With an increasing ZnO/SiO₂ molar ratio, the band at 622 cm⁻¹ is shifted towards 645 cm⁻¹ and finally, splits into two components for 10 mol% ZnO content glasses.²⁰³

Fig. 16 Raman spectra of glasses for glass system 5Na₂O·20CaO·5Al₂O₃·(60 − x)SiO₂·xZnO²⁰³ (reproduced with permission from Elsevier).

Sodium silicate and aluminosilicate can be analyzed by Raman spectroscopy in the low frequency region of 20–250 cm⁻¹. In this region, the peak at 69 cm⁻¹ is the signature of Raman scattering involving rotational motions of interconnected tetrahedral units. The position of this peak is shifted to 78 cm⁻¹ with the addition of Al₂O₃ up to 6 mol%. The Raman peak at 170 cm⁻¹ is assigned to a narrower distribution of T–O–T angles or to a structural localization of those vibrations in specific clusters of molecules in the glass network. The band centred at 490 cm⁻¹ is shifted to 482 cm⁻¹. The band at 597 cm⁻¹ becomes broader with an increase in the content of Al/(Al + Na). This result suggests the presence of three-membered rings in the glass network. The Raman band located at 781 cm⁻¹ is shifted to 799 cm⁻¹ with replacement of Na₂O by Al₂O₃. A shoulder near 810–820 cm⁻¹ is a feature of Si–O stretching involving oxygen motions in the Si–O–Si plane or of the motion of the Si atom in its oxygen cage or a threefold-degenerate rigid cage vibrational mode of TO₂ units. The high frequency bands at 960, 1070, 1100, 1150 and 1200 cm⁻¹ are characteristic of silicate networks and vibrational modes of TO₄ tetrahedra.²⁰⁴ A Raman band at 495 cm⁻¹ was observed in albite glass and showed the presence of polymeric structures in such glasses. The band at 595 cm⁻¹ is characteristic of structural defects in the glass involving T–O (non-bridging) vibrations (Fig. 17). The defect line, D₂, of SiO₂ is at 606 cm⁻¹. Al in borosilicate glass shifted the band positioned at 950 cm⁻¹ to 900 cm⁻¹, while peaks at 980 and 1110 cm⁻¹ showed the opposite trend to the previous result. The polymerization becomes more significant with an increasing Al/Na ratio.²⁰⁵

Fig. 17 Raman spectra of 75SiO₂·(25 − x)Na₂O·xAl₂O₃ glasses²⁰⁴ (reproduced with permission from Elsevier).

The structure of aluminosilicate glasses consists of silicate and aluminate networks. The bands centred at 1060 and 1200 cm⁻¹ are attributed to Si–O stretching vibrations of SiO₄ tetrahedra. The intensities of bands at 1000 and 1100 cm⁻¹ are increased with an increase in Al₂O₃. This increase in the intensity of these bands is due to the interlinking of the Si–O–Al network. The intensity of the band at 435 cm⁻¹ was decreased with an increase in Al₂O₃ and this was believed to be the result of a decrease of Si–O–Si linkages in tetrahedral units.²⁰⁶ Aluminosilicate glasses along with silica–calcium, silica sodium aluminate and silica–potassium aluminate showed the appearance of bands centred at 1120, 1100, 930 and 890 cm⁻¹. The degree of metal cation induced network clustering increases in the order K < Na < Li < Ca < Mg within the SiO₂–MAlO₂ glasses (M = K, Na, Li, Ca and Mg).^207,208 The low frequency bands at 450, 500 and 600 cm⁻¹ are associated with the motions of bridged oxygen in T–O–T linkages and the band at 560 cm⁻¹ in the Al-rich sample is caused by the presence of Al–O–Al bridges in the CaO–Al₂O₃–SiO₂ system. The intensity of a band at 800 cm⁻¹ decreased with a decrease in SiO₂ and shifted towards a lower wavenumber. The high frequency Raman spectra consists of peaks at 1000 and 1080 cm⁻¹. Al coordination exists mainly in four-fold type and varies between Q²_Si and Q⁴_Si species as a function of the MO/Al₂O₃ ratio (M = Ca, Sr, Ba). At much higher concentrations of CaO (more than 62 mol%), a Raman band between 300 and 400 cm⁻¹ was also present. A Raman band at 170 cm⁻¹ and a shoulder around 300 cm⁻¹ was observed in Ba containing glasses. The intensity of the peak at 170 cm⁻¹ increases with Ba content. The peak position of Raman bands due to metal cations decreases with the size of the cation.^209–212 A Raman band located at 450 cm⁻¹ is characteristic of the motion of bridging oxygens in the plane perpendicular to the Si–O–Si (A1) bond. The vibration at 710 cm⁻¹ provides evidence of AlO₄ tetrahedra and the band at 760 cm⁻¹ corresponds to the stretching mode of the Al–O bond with aluminium in four-fold coordination. The band at 960 cm⁻¹ in the spectra is caused by stretching vibrations of silicon–oxygen tetrahedra with two corners shared with aluminium–oxygen or calcium–oxygen polyhedra.²¹³ The depolarization ratios in calcium alumino-silicate glasses are 0.007 and 0.485 for intense bands at 540 and 96 cm⁻¹, respectively. This result showed that the vibration at 540 cm⁻¹ is highly symmetric in nature. The polarized spectrum for this glass system showed peaks centred at 96, 540, 767 and 867 cm⁻¹. Similar bands are observed in other glass system with a slight variation in their peak positions.²¹⁴ The main peak assignments in the Raman spectra of aluminosilicate glasses are listed in Table 6.

Table 6 Assignment of Raman bands in the spectra of Aluminosilicate glasses

Wavenumber (cm^−1)	Raman assignments	Reference
1060, 1200	Si–O stretching vibrations of SiO4 tetrahedra	206
960	Stretching vibration of silicon–oxygen tetrahedra with two corners shared with aluminium–oxygen or calcium–oxygen polyhedra	213
810–820	Si–O stretching involving oxygen motions in the Si–O–Si plane or the motion of the Si atom in its oxygen cage or the threefold-degenerate rigid cage vibrational mode of TO2 units	204
779	Si–O–Si network, ZnO4 tetrahedra and AlO4 units with three BOs and one NBO	203
450, 500, 600	Motions of bridged oxygen in T–O–T linkages	209–212
400–650	Bending vibrations of the bridging oxygen (BO) bonds of SiO	203

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3.7. Phosphosilicate glasses

A typical Raman spectrum of phosphosilicate glass is shown in Fig. 18.²¹⁵ Raman spectra of phosphosilicate glasses consist of networks of phosphate as well as silicate. A Raman peak at 1320 cm⁻¹ is attributed to the stretching vibrations of O–P double bonds and the bands below 860 cm⁻¹ are assigned to Si–O and P–O bonds in Si–O–Si, P–O–Si and P–O–P linkages. The peaks at 1200 and 1020 cm⁻¹ are caused by P–O–P linkages and the band near 1145 cm⁻¹ is due to P–O–Si linkages.^215,216 The bands are located at 340, 426, 580, 648, 720, 780, 882, 956, 1040 and 1450 cm⁻¹ in the Raman spectra of various compositions of a CaO–MgO–SiO₂ system doped with B₂O₃, P₂O₅, Na₂O and CaF₂ (Fig. 19).

Fig. 18 Raman spectrum of phosphosilicate glass²¹⁵ (reproduced with permission from Elsevier).

Fig. 19 Raman spectra of calcium magnesium silicate glasses doped with B₂O₃, P₂O₅, Na₂O and CaF₂ (ref. 217) (reproduced with permission from Elsevier).

The vibrations in the region 800–1300 cm⁻¹ are characteristic of the asymmetric vibration of SiO₄ tetrahedra, whereas the bands at 952, 590 and 425 cm⁻¹ are assigned to the symmetric stretching of the P–O⁻ bonds and O–P–O bending modes of the orthophosphate PO₄³⁻ unit (Q²_P), respectively.²¹⁷ The silicate networks Si–O–Si and the intensity of the Si–O–NBO stretching mode are modified by cations Na, Mg and Ca.^218,219 The addition of silver to calcium phosphosilicate glasses changes some networks, for example the contribution of Q⁰_P, Q³_P and Q⁴_P progressively vanishes as a result of increasing the concentration of silver and a weak band around 1025 cm⁻¹ also appears. The position of a band at 956 cm⁻¹ was shifted to 967 cm⁻¹ with this addition. The spectra also show the occurrence of a depolymerization process of the SiO₄ network.²²⁰ The average distance between silicates, phosphates, inter silicate–phosphate and aluminium–phosphate chains was increased by increasing the concentrations of AlO₆ structural units and this leads to an increase in the average bond lengths of Tm–O and Er–O due to weaker fields around Tm–O and Er–O ions.²²¹ The main peak assignments in the Raman spectra of phosphosilicate glasses are listed in Table 7.

Table 7 Assignment of Raman bands in the spectra of phosphosilicate glasses

Wavenumber (cm^−1)	Raman assignments	Reference
1320	Stretching vibrations of O–P double bond	215 and 216
1200, 1020	P–O–P linkages	215 and 216
1145	P–O–Si linkages	215 and 216
860	Si–O and P–O bonds in Si–O–Si, P–O–Si and P–O–P linkages	215 and 216

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3.8. Alumino-borosilicate glasses

Raman spectra of NaF alumino-borosilicate glasses consist of vibrations located at 1075, 947, 802, 723 and 480 cm⁻¹ (Fig. 20). The Raman band located at 802 cm⁻¹ is characteristic of Si–O–Si vibrations and symmetric breathing vibrations of the boroxol rings. Sodium fluoride (NaF) in alumino-borosilicate glass increases the stability of the boroxol ring. The band at 723 cm⁻¹ is assigned to the metaborate group, whereas lower intensity bands centered at 1075, 947 and 480 cm⁻¹ are attributed to the stretching vibrations of Si–O–Si and B–O–Si. Instead of the abovementioned bands, a few more bands at 1114, 1000, 885, 770, 657, 598, 520, 491 and 447 cm⁻¹ also appear in the Raman spectrum. The band located at 1114 cm⁻¹ is attributed to diborate groups. The band at 885 cm⁻¹ indicates the presence of pyroborate groups. The existence of symmetric breathing vibrations of six-membered rings in the BO₄ tetrahedron was confirmed by a band located at 770 cm⁻¹. The bands at 1000 and 520 cm⁻¹ indicate the availability of the vibrations of Si–O⁻, B–O–B, B–O–Si and Si–O–Si bending. The rocking vibration is also found in a network of glass at 475 cm⁻¹. A Raman band at 951 cm⁻¹ is attributed to the B–O–Si stretching vibration. The shoulder peak at 447 cm⁻¹ is assigned to the bending or rocking vibrations of B–O–Si linkages, B–O–B, B–O–Si and Si–O–Si.²²²

Fig. 20 Raman spectra of NaF alumino-borosilicate glasses²²² (reproduced with permission from Elsevier).

Raman spectroscopy of sodium-alumino-borosilicate glasses in the presence of Gd₂O₃ showed that the band at 1050 cm⁻¹ is shifted towards a lower wavenumber with the addition of Gd₂O₃ and the intensity of the band centred at 450 cm⁻¹ was increased. One more band at 300 cm⁻¹ appeared in glasses with 7.06 and 13.58 mol% of Gd₂O₃. The borate network was much influenced by network modifying cations as compared to the silicate network. Gd cations are more effective than Na cations with respect to the modification of networks.²²³ Raman spectra of strontium alumino-borosilicate glasses exhibit vibrations of borate as well as silicate networks along with some linking networks. Asymmetric ring breathing vibrations are characterised by a band at 430 cm⁻¹, symmetric stretching vibrations of Si–O–Si by a band at 800 cm⁻¹, stretching vibrations associated with SiO₄ composed of sites with three and four-fold rings ranging from 1000 to 1200 cm⁻¹ and symmetric stretching vibrations are characterised by a band at 1200 cm⁻¹. The borate networks in pure B₂O₃ exhibit a strong band at 805 cm⁻¹ due to the symmetric beating vibrations of boroxol ring oxygens. The boroxol ring structure is split into different ring structures (i.e. triborate, tetraborate or pentaborate) in the presence of suitable modifiers in the glass network (Fig. 21). After irradiation with γ-rays, the intensity of bands due to asymmetric vibrations increased and shifted to a higher wavenumber. The effect of radiation was less in Al₂O₃ containing glasses.²²⁴ Not only γ-ray irradiation but also β-ray irradiation changes the network of the glasses. The bending vibration of Si–O–Si at 460 cm⁻¹ is shifted in glasses irradiated with 10⁹ Gy. β-Ray radiation decreases the ratio of Q²_Si species to Q³_Si ones.^225,226 β-Ray irradiation of rare earth alumino-borosilicate glasses reduces the number of Cr and Mn ions in the matrix. The Raman band at 460 cm⁻¹ is shifted to 480 cm⁻¹ with addition of Cr instead of Sm and Gd. Silicate networks in the range 900–1200 cm⁻¹ are not significantly affected by Cr, whereas new bands centered at 845 and 900 cm⁻¹ are introduced in the spectrum. The behaviour of the Raman band at ∼1550 cm⁻¹ due to molecular oxygen could not be analyzed under irradiation due to polishing of the glass surface.^226–229 As with β-ray irradiation, γ-irradiation is effective in reducing Sm³⁺ to Sm²⁺ ions with a 10 kGy dose.²³⁰ The Raman band assignments in alumino-borosilicate glasses are listed in Table 8.

Fig. 21 Raman spectra of glass 40SrO–5Al₂O₃–15B₂O₃–40SiO₂ irradiated with doses of (1) 0 kGy; (2) 10 kGy; and (3) 30 kGy (ref. 224) (reproduced with permission from Elsevier).

Table 8 Assignment of Raman bands in the spectra of alumino-borosilicate glasses

Wavenumber (cm^−1)	Raman assignments	Reference
1114	Diborate groups	222
800–1300	Asymmetric vibration of SiO4 tetrahedra	222
1000, 520	Vibration of Si–O^−, B–O–B, B–O–Si and Si–O–Si bending	222
952	Symmetric stretching of the P–O^− bonds	222
951	B–O–Si stretching vibration	222
885	Pyroborate groups	222
770	Symmetric breathing vibration of six-membered rings, one BO4 tetrahedron	222
590, 425	O–P–O bending modes of the orthophosphate PO4^3− unit	222
447	Bending or rocking vibrations of the B–O–Si linkages, B–O–B, B–O–Si and Si–O–Si	222

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3.9. Tellurite glasses

The Raman spectra of various tellurite glasses are shown in Fig. 22. In tellurite glasses, tellurium oxide works as a glass former. Tellurite oxide has three basic structures, namely, TeO₄ (trigonal bipyramid, tbp), TeO₃ (pyramid) and an intermediate with a TeO_3+δ polyhedron. The four oxygen atoms are covalently bonded with a central tellurium atom in TeO₄. The bipyramid structure is formed by two equatorial and two apex oxygen sites. The trigonal pyramid structure consists of two bridging oxygen sites and one non-bridging oxygen atom.^231,232 The Raman bands near 667 cm⁻¹ are related to the combined vibrations of asymmetric stretching of Te–_eqO_ax–Te bonds and symmetric stretching of TeO₄ tbps. The addition of Na₂O to the glassy matrix changes the intensity of bands due to stretching vibrations of non-bridging Te–O⁻ bonds in TeO₃ tps (753 and 792 cm⁻¹). The position of a band assigned to the symmetric bending vibration of TeO₄ tbps is shifted towards a lower wavenumber. The intense band at 40 cm⁻¹ is associated with a Boson peak. An increase in the Na₂O content of tellurite glasses results in the transformation of TeO₄ tbps to TeO₃ tps due to an increased number of NBOs.^233,234 Assignments of the main Raman bands in the spectra of tellurite glasses are listed in Table 9.

Fig. 22 Raman spectra of various tellurite glasses²³¹ (reproduced with permission from Elsevier).

Table 9 Assignment of main Raman bands in the spectra of tellurite glasses

Wavenumber (cm^−1)	Raman assignments	Reference
40	Boson peak	233 and 234
60–80	β-TeO2	239
110–143	γ-TeO2	239
300	TeO3	236
440–478	Stretching and bending vibration of Te–O–Te linkages in TeO4 (tbps), TeO3+δ polyhedra and TeO3 (tps)	231,235,236,243 and 246
581–623	Vibration of the continuous network comprised of TeO4 tbps	231
648–700	Antisymmetric vibrations of Te–O–Te in TeO3+δ, TeO4, TeO3 and TeO4 networks	231,233,234,239 and 250
716–753	Stretching vibrations between tellurium and non-bridging oxygen (NBOs) atoms	231,233,234,239 and 250
773–793	Continuous network vibration of TeO4 tbps and a TeO^− stretching vibration of TeO3+δ polyhedra or TeO3	231,233,234,239 and 250

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The study of xNa₂O·(35 − x)V₂O₅·65TeO₂ glass showed an increase in the intensities of TeO₄ tbp (443–478 cm⁻¹ and 671–675 cm⁻¹) and TeO₃ tp (783–793 cm⁻¹) with an increase in the Na₂O content. This result is also consistent with the abovementioned result of the transformation of TeO₄ tbps to TeO₃ tps.²³⁵ The network Te–O–Te bridges are broken by additional network modifiers, Na₂O, ZnO and ZnF₂, in tellurite and form the non-bridging oxygens. The intensity of vibrations of TeO₃ (300 cm⁻¹) and Te–O–Te (470 cm⁻¹)/TeO₄ (675 cm⁻¹) are reduced by the replacement of ZnO with ZnF₂ in the glassy matrix. The tellurium chain was broken by the introduction of P₂O₅ to tellurite glasses, which suggests the formation of Te–O–P bonds.²³⁶ Raman peaks of crystallized sodium tellurite glasses are very sharp and intense. The peak positions were shifted towards a lower wavenumber with an increase in the crystallization temperature.²³⁷ The network of TeO₄ bipyramids are broken to form networks of TeO₃₊₁ and TeO₃ polyhedra at high pressures.²³⁸ Raman spectra of zinc tellurite glasses confirm the presence of β-TeO₂ (60–80 cm⁻¹) and γ-TeO₂ (110–143 cm⁻¹). The three dimensional network of asymmetrical vibrations of TeO₄ units with one lone pair of electrons at an equilateral position is linked to Te_ax–O_eq–Te linkages. The formation of Zn₂Te₃O₈ units changes the bands positioned at 295, 343 and 700–800 cm⁻¹.^239–241 The peak positions of bands at 423 cm⁻¹ is found to shift towards a lower wavenumber with increasing ZnO content, whereas bands at 661 and 720 cm⁻¹ are shifted towards a higher wavenumber with increasing ZnO concentration. The intensities of bands at 661 cm⁻¹ (decrease) and 720 cm⁻¹ (increase) are reversed in order with an increase of ZnO content from 18 to 35 mol%. This indicates a decrease in Te–O coordination due to the conversion of TeO₄ into TeO₃ structural units with an increase in ZnO content.²⁴² A Raman study of TeO₂–La₂O₃–TiO₂ glasses shows the chain formation of Te–O–Ti–O–Te– at a lower content of TiO₂, whereas the chain formation becomes saturated and some TiO₄ polyhedra are also formed in the glassy matrix.²⁴³ The structural change from TeO₄ trigonal bipyramids (tbps) to TeO₃ trigonal pyramids (tps) via [TeO₃₊₁] is also observed with an increasing Ta₂O₅ content in glass.²⁴⁴ The Raman spectra of (89 − x) TeO₂–10TiO₂–1Nd₂O₃–xWO₃ indicates the depolymerisation of tellurite glass as the Te–O–Te inter-chain linkages are progressively substituted by stronger Te–O–W bridges as a result of the addition of WO₃.²⁴⁵ The addition of Nb₂O₅ in tellurite glasses decreases the connectivity by the deformation of Te–O–Te linkages.²⁴⁶ The network modifying nature of K⁺ ions is greater than that of Li⁺ ions in the alkali metal tellurite glasses due to the larger ionic radius of potassium compared to that of lithium.²⁴⁷ The thallium ions in tellurite glasses induce the island-like structure of [TeO₃]²⁻.²⁴⁸ The alkali ion (Li and Na) transports drive the secondary event as NBO migration through BO–NBO switching.²⁴⁹ Khanna et al. studied the Raman spectra of boro-tellurite and aluminoboro-tellurite glasses. A Raman spectrum of boro-tellurite glass depicts bands at 450, 503, 615, 665, 718, 762, and 902 cm⁻¹, whereas alumino-boro-tellurite glasses have Raman bands located at 337, 450, 505, 567, 606, 666, 724, 754 and 864 cm⁻¹.²⁵⁰ The structural units TeO₄ tbps and TeO₃₊₁ polyhedra change into TeO₃ tps with an increase in the B₂O₃ content.²⁵¹ The diamond in tellurite glasses is a good medium for quantum information. The first order diamond Raman band is at 1350 cm⁻¹ in nano-diamond induced tellurite glasses.²⁵²

4. Conclusions

A detailed study of Raman spectroscopy data on various alkali and alkaline earth metal doped oxide glasses is reviewed. The various models were helpful in the accurate correlation of data. A general agreement was reached that the structure of glassy networks is composition dependent. The vibrational properties of structural units are better understood with respect to their structure and bonding as vibrational spectroscopy cannot be used as a tool for analyzing their detailed molecular structure. Raman spectroscopy studies clarify the modification of vibrational networks of borate by alkali and alkaline earth metals. The basic borate networks are formed by NBO and BO atoms in a tetrahedral borate unit.⁹ The formation of pentaborate, tetraborate, diborate, metaborate and pyroborate units¹⁰ and the disappearance of diborate units,¹⁰² boroxol rings¹¹⁶ and high polymeric units such as pentaborate¹⁰⁵ caused by certain dopants was clearly noted by Raman spectroscopy studies of various borate glasses. Raman spectroscopy of oxide glasses showed that the modifying nature of alkali and alkaline earth metals are in the order Cs < K < Na < Li < Ca < Mg.²³ The alkaline metals are effective in the order Ba < Sr < Ca < Mg, whereas the alkali metals are in the order K < Na < Li. Significant information regarding the glass forming ability of various glassy networks was obtained from Raman spectroscopy studies in the present review article. On the basis of the abovementioned investigations of oxide glasses, dopants MoO₃, CeO₂, Bi₂O₃, Sb₂O₃ and TeO₂ form mainly MoO₆ or MoO₄, CeO₈, BiO₃ or BiO₆, SbO₃ and TeO₃ or TeO₄ units, respectively. Raman studies also provided significant information about UV, β and γ-ray irradiation. It was found that these radiations decrease the Q²_Si species compared to Q³_Si species and reduce the oxidation state of cations.²³⁰ Tellurium oxide works as a network former as well as a network modifier. With the addition of alkali metal oxides, TeO₄ tbps units transform into TeO₃ tps due to an increased number of NBOs.

Great advances have been seen in the structural studies of glasses in the past few decades; however, a few questions remain open to study. The presence or absence of super-structural units in the glass network is the most important question. The mesounits in an intermediate- or medium-range order in the oxide structure affect a variety of the properties of the glass and are constituted by 7–20 or more atoms. The primary information about mesounits and their possible arrangements can be derived from the study of crystal structures that are isocompositional with the glasses. Such types of information could be obtained from Raman spectroscopy. Quantitative information on structural units in glasses may also be obtained by Raman spectroscopy analysis. It is always a question whether information obtained by Raman spectroscopy is complete or not. The vibrations, which are antisymmetrical with respect to the centre, are Raman inactive. In this case, researchers might question whether anti-symmetric vibrations can be analysed by Raman spectroscopy. Such vibrations are analysed by infrared spectroscopy. What do the analogies with organic high polymers teach us with respect to the glass? In what respect can a given glass be called a polymer?

The absence of a change of P-coordination in phosphate glasses as found in borate is not explained by the structural study of phosphate glasses by Raman spectroscopy. No particular relationship is seen between the distribution of tetrahedral linkages and the glass properties. A question also arises whether there are any differences between the non-bridging oxygens on individual Q²_P and Q¹_P tetrahedra.

Acknowledgements

The authors are thankful to the publishers Elsevier, Mineralogical Society of America, Springer, IOP science and EBSCO Publishing for permitting the reproduction of cited contributions for academic purposes in this review article. The authors acknowledge the authors of cited articles. The funding agency CSIR, New Delhi, (India) is gratefully acknowledged for financial support under project No. 03(1300)/13/EMR-II.

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