Influence of lanthanum on structural and magneto-optic properties of diamagnetic glasses of the TeO₂–WO₃–PbO system

Abstract

This paper is focused on the design, fabrication and characterization of tellurite glass of composition TeO₂–WO₃–PbO in terms of La₂O₃ addition. The effect of La₂O₃ on the structure of the obtained glasses has been investigated by means of spectrophotometric and ellipsometric measurements in the UV-Vis-NIR spectral region and then relevant optical parameters have been calculated. The DC magnetic susceptibility study has been used to investigate the influence of lanthanum ion (La³⁺) on the structural and magnetic properties of the investigated glass system. Based on DTA, PALS and Raman studies, it can be stated that modification of the basic structural units with La³⁺ ions, namely the TeO₄ trigonal bipyramid (tbp) and the TeO₃ trigonal pyramid (tp), both having a lone pair of electrons occupying one of the equatorial positions, affect the magnetic properties of these glasses.

---

1. Introduction

The structure and properties of tellurite glasses are strongly dependent on the nature and concentration of the constituent oxides, resulting in considerably increased interest due to their possible potential applications in photonics, optoelectronic applications and also as good candidates for ultrasonic materials.^1–4

This type of glass has some remarkable features such as low melting temperature and, in comparison to halide and sulfide glasses, relatively low phonon energy, high refractive index, high dielectric constant and good corrosion resistance.^5,6 Thermal properties of tellurite glasses from the TeO₂–WO₃–La₂O₃ and TeO₂–WO₃–PbO have been extensively studied by D. Sushama and P. Predeep.⁷ They have found that tellurite glasses containing La₂O₃ are thermally more stable than the tellurite glasses containing PbO. The positive influence of lanthanum oxide addition on the thermal stability of glasses of the TeO₂–WO₃–PbO–La₂O₃ system has been confirmed by earlier papers.^8,9

To improve the thermal stability of very well known glass system TeO₂–ZnO–Na₂O,^10–12 the heavy metal oxides such as BaO and La₂O₃ have been added.¹³ La³⁺-ion containing glasses are reported as excellent laser materials and presence of these ions in the glass host greatly improve the nonlinear optical properties of glasses. La³⁺ is paramagnetic ion and is being often used to test the glass structure since the outer f-electron orbital function has a broad radial distribution.^14–16

These ions when dissolved in glass matrices have a strong influence on their optical and magnetic properties. Thus, taking into account the aforementioned issues, the effect of La₂O₃ on the structure and properties of tungsten–tellurite glasses has been investigated by means of spectrophotometric and ellipsometric measurements in the UV-Vis-NIR region and then the relevant optical parameters have been calculated. The DC magnetic susceptibility study has been used to investigate the influence of lanthanum ion (La³⁺) on the structural and magnetic properties of 60%TeO₂–27%WO₃–10%PbO–3%La₂O₃ glass system.

2. Experimental

The glasses, with the molar composition 60%TeO₂–30%WO₃–10%PbO (T1) and 60%TeO₂–27%WO₃–10%PbO–3%La₂O₃ (L1), were obtained by melting 20.00 g batches of the high-purity (99.99%) chemicals in gold crucibles in an electric furnace at 850 °C in the air atmosphere. The crucibles were covered with a platinum plate to avoid vaporization losses. Then the melts were poured onto plates preheated to 400 °C, forming a few mm thick layers, subsequently annealed in the temperature range 320–340 °C. The PALS measurements were performed at room temperature using a conventional fast–fast coincidence system with an ORTEC.¹⁷ The time resolution of the system was 270 ps (full width at half maximum). A Na²² isotope positron source of 10⁵ Bq activity was situated between two identical samples, forming a ‘‘sandwich’’ system. Analysis of the PALS spectra was carried out with the use of the LT computer programme.¹⁸ An extensive description of this method may be found in.¹⁹

The Raman spectra were recorded using a WITec confocal CRM alpha 300 Raman microscope equipped with an air-cooled solid state laser operating at 488 nm a CCD detector. A dry Olympus MPLAN (1006/0.90NA) objective was used. The power of the laser at the sample position was between 14.4 and 14.6 mW for a measurement. 120 or more scans with integration times of 0.3–0.5 s and a resolution of 3 cm⁻¹ were collected and averaged. For spectroscopic measurements, the glass samples were sliced and polished to dimensions of about 10 × 10 × 2 mm³. The ellipsometric data were obtained with a M-2000 Woollam ellipsometer,²⁰ while the transmittance and reflectance spectra were recorded with a Perkin Elmer Lambda 900 spectrophotometer.

The refractive index dispersion of tellurite glasses is very well described by the Sellmeier model. Therefore our ellipsometric data, gathered as a function of wavelength λ, have been fitted in the range of low absorption, i.e. 400–1700 nm, to the Sellmeier refractive index (n) normal dispersion of the form: n² = A + Bλ²/(λ² − C²) − Dλ².²¹

The experiment probing the Faraday effect has been carried out in a magnetic field with an induction of B = 0.06 T. The measurements of steering angle has been executed in a range 500–600 nm. The value of Verdet constant V was calculated from the formula θ = VlH. The angle θ is proportional to the light path through the glass and the magnetic field strength H. The Verdet constant depends on the magnetic properties of the glass. The block schema of measuring system may be found in.²²

The DC magnetic susceptibility was used to investigate the influence of lanthanum ion (La³⁺) on the structural and magnetic properties of TeO₂–WO₃–PbO glass system. The room temperature magnetization curves were measured by LakeShore 7307 vibrating sample magnetometer at external magnetic field up to 2 T. Magnetic moments for all samples were determined by subtraction of the magnetic response from the sample holder. All the measurements were performed on the amorphous glasses, what was confirmed by the XRD method.

3. Results and discussion

Fig. 1 shows the Raman spectra of the glass samples (T1): 60TeO₂–30WO₃–10PbO and (L1): 60TeO₂–27WO₃–10PbO–3La₂O₃ mol%. The Raman spectrum of glass T1 consists of four bands at 911, 728, 473 and 347 cm⁻¹. The Raman studies on tungsten tellurite glasses have shown that observed band at 911 cm⁻¹ is assigned to symmetric stretching vibrations of W–O⁻ and W=O bonds associated with WO₆ units.²³

Fig. 1 Raman spectra of tellurite glasses.

The Raman bands at 858 cm⁻¹,²⁴ assigned to stretching vibrations of W–O–W in WO₄ or WO₆ units is not observed. It might appear after the deconvolution of the spectra. The band at 728 cm⁻¹ can be assigned to symmetric stretching vibrations in distorted trigonal bipyramidal (TeO₄) unit between Te and non-bridging oxygen (NBO) of TeO₃₊₁ units, or to a Te=O stretching bond in O=TeO₂ units.³

The Raman band at 473 cm⁻¹ can be attributed to the stretching vibrations of Te–O–W or Te–O–Te linkages. The band at 347 cm⁻¹ may be attributed to bending vibrations of W–O–W in WO₆ units.²⁵ Substitution of WO₃ by La₂O₃ causes a minor change in the Raman spectra of tungsten–tellurite glasses. The intensity of all bands decreases and a new band at 664 cm⁻¹, assigned to symmetric stretch of Te–O in TeO₄ units, is observed in addition to that of 728 cm⁻¹. It leads to a decrease of Te–O bonds in the glass network with non-bridging oxide (NBO) and the formation of La–Te–O, La–O–Te or O–La–O bonds.²³

The changes of structure of tellurite glasses with La₂O₃ addition was also confirmed by the positron annihilation lifetime spectroscopy (PALS). It was found that the best match was obtained at the spectrum distribution into two components, namely τ₁ and τ₂ lifetimes and corresponding intensities I₁ and I₂ (Table 1). As a result of the measurements, no τ₃ component, i.e. the component responsible for creation of positronium in the studied materials (hydrogen-like atom), i.e. formation of free volumes of the size of double diameter of a hydrogen atom, was found.²⁶ To calculate physical parameters describing the defective state in tellurite glasses, a two-state model was used, according to which a positron annihilates from the free state and from one state localised in a defect, in absence of the detrapping process.²⁷ By accepting two-state positron trapping model with only one kind of such defects, the numerical parameters of positron trapping (mean τ_av and defect-free bulk τ_b positron lifetimes, and positron trapping rate in defects κ_d) can be calculated from the well known formulae:

Table 1 Parameters of positron lifetimes and positron capture

Sample	τ1 [ns]	I1 [%]	τ2 [ns]	I2 [%]	τav [ns]	τb [ns]	κd [ns^−1]	τ2 − τb [ns]	τ2/τb [ns]
T1	0.259 ± 0.008	61.86 ± 0.87	0.405 ± 0.012	38.14 ± 0.54	0.315	0.300	0.531	0.105	1.349
L1	0.274 ± 0.008	72.12 ± 0.95	0.423 ± 0.013	27.88 ± 0.41	0.316	0.304	0.358	0.119	1392

---

In addition, the difference (τ₂ − τ_b) can be calculated as a signature of size of extended defects which trap positrons in terms of equivalent number of monovacancies, whereas (τ₂/τ_b) ratio is ascribed to the nature of these defects.²⁸ The calculated positron capture parameters τ_av, τ_b, κ_d, τ₂ − τ_b and τ₂/τ_b are also listed in Table 1.

It is evident from the measurements of positron lifetime that the τ₁ and τ₂ values of positron lifetimes are slightly different, within the limits of error, for both glasses T1 and L1 (similar for τ_av and τ_b parameters). However, a significant change of intensity values of the I₁ and I₂ components and positron capture parameters κ_d, τ₂ − τ_b, τ₂/τ_b can be observed.

Taking into consideration the positron capture parameters (Table 1), we can assume:

Slightly different values of τ₁, τ₂, τ_av and τ_b shows that the kind and type of place of annihilation acts and defects occurring in both T1 and L1 glasses of the same type.

The rate of positron capture κ_d in sample L1, as compared to sample T1, gradually decreases. This is evidence of much lower concentration of volume defects and positron capture centres in sample L1 (influence of La₂O₃ addition).

Parameter τ₂ − τ_b indicates a insignificant difference of the examined samples of tellurite glasses, meaning that average sizes of defects in which positron captures occur in sample with La₂O₃.

The τ₂/τ_b increases in sample L1 that is an evidence of different geometry of volume defects. Places of positron capture have a different nature.

Addition of La₂O₃ (sample L1) resulted in decrease of concentration of positron traps and volume effects. However, there occurred an increase of average defect sizes and the geometry of volume effects was also changed. Resolution of electron density in the examined tellurite glass with La₂O₃ decreases significantly.

The decrease of NBO in the investigated glass, confirmed by the structural studies, also affects its magneto- and optical properties.

From the Fig. 2, it can be seen that the UV-Vis-NIR transmission in the studied glasses slightly decreases with the substitution of WO₃ by La₂O₃. The transmittance of both glasses T1 and L1 in the range of 900–2500 nm, is similar – about 75%, whereas the glass T1 exhibits a local minimum of transmission at about 700 nm. Reflectance spectra of both glasses are also very similar.

Fig. 2 Transmittance and reflectance spectra of tellurite glasses.

Moreover, the refractive index n ≈ 2 calculated according to the formula:

is in good agreement with the ellispometric data.

Additionally, as shown in Fig. 2, the sum of transmission and reflection (T + R) for both glasses in the transparent region is about 98%, indicating that the scattering of light by the surface and volume defects is very small, and the glasses studied are optically homogeneous.

The refractive index of tellurite glasses, within the 400–1700 nm spectral range, presented in Fig. 3, exhibits very high values, over 2.1 that is considerably higher than those obtained for standard optical glasses.²⁹

Fig. 3 Dispersion of refractive index of glass samples TWP (T1) and TWPLa (L1) obtained from ellipsometric measurements.

Values of the Sellmeier parameters A, B, C and D along with the surface roughness determined from the fitting procedure are presented in the Table 2.

Fig. 4 shows the absorption coefficient as a function of photon energy for the investigated glasses. The absorption edge of T1 and L1 glasses occurs near the ultraviolet region.

Fig. 4 Absorption coefficient as a function of photon energy.

Table 2 Sellmeier parameters and roughness obtained from ellipsometric studies

Sample	A	B	C [μm^2]	D [μm^−2]	n at 633 nm	Roughness [nm]
TWP (T1)	2.898	1.651	0.25401	0.02251	2.204	5.7
TWPLa (L1)	2.793	1.639	0.24887	0.02182	2.173	9.7

---

The absorption edge of these glasses is determined by the oxygen bond strength in the glass-forming network. Thus the addition of La³⁺ to TeO₂ leads to the replacement of the O–Te–O, and/or Te–O–Te bands in the glass network by La–Te–O, La–O–Te and/or O–La–O bands which is reflected in the absorption spectra by a slight shift of the absorption edge to shorter wavelengths (higher energy). The shift of the absorption edge are most likely related to the structural rearrangement of the glass. Any change of oxygen bonding in the glass network, for instance, the formation of nonbridging oxygen, changes the characteristic absorption edge.³²

The magnetic susceptibility measurement technique offers useful information concerning the nature of the magnetic interactions between La³⁺ ions inside the host glass matrix.^30,31 The field dependence of magnetization of the investigated glasses is shown in Fig. 5.

Fig. 5 Magnetization versus magnetic field for the glasses T1 and L1.

The magnetic susceptibility measurements allow us to determine the dominant type of paramagnetic and diamagnetic arrangement as a function of changes in the external magnetic field. When comparing the magnetic properties of the parent glass (T1) and the glass with La₂O₃ addition (L1) we see a significant change in magnetic ordering. Parent glass T1 is a typical diamagnetic. The addition of lanthanum to the parent glass causes the change in magnetic ordering from diamagnetic to paramagnetic. On the other hand it can be stated that this change can be seen only in the range of low magnetic fields.

For glass T1 the dependence of the turn angle of the plane of polarization on the light wavelength under magnetic field was not observed. Such a dependence is typical for diamagnetic.³³ The well visible Faraday effect has been registered in glass L1 and may be connected with La₂O₃ addition (Fig. 6). The maximum value of the turn angle was obtained for the wavelength λ = 510 nm. The high value of Verdet constant (25.36 × 10⁴ min T⁻¹ m⁻¹) is the result of the change of the magnetic ordering of glass from diamagnetic to paramagnetic.

Fig. 6 The dependence of the Verdet constant from wavelength in constant magnetic field of induction 0.06 T for the L1 glass.

4. Conclusions

Transparent and stable glasses were obtained in the TeO₂–WO₃–PbO, and TeO₂–WO₃–PbO–La₂O₃ glass systems. The Raman spectra of glasses reveal that the glass network consists of TeO₄, [TeO₃]/[TeO₃₊₁] units. The lanthanum addition leads to the decrease of Te–O bonds in the glass network with non-bridging oxide (NBO). The UV-Vis-NIR transmission in the studied glasses slightly decreases with the substitution of WO₃ by La₂O₃. The optical absorption spectra show that, the absorption edge shifts to higher wavelengths when La³⁺ ion is added into the glass structure.

Much lower concentration of volume defects and positron capture centres in the glass L1 have been registered. Addition of La₂O₃ (sample L1) results in decrease of concentration of positron traps and volume effects.

The high value of the Verdet constant (25.36 × 10⁴ min T⁻¹ m⁻¹) is the result of the change of the magnetic ordering of glass from diamagnetic to paramagnetic. It is clear that the paramagnetic susceptibility of the investigated glasses increases with the increase of the rare earth ions (La³⁺ ion content).

References

1. R. A. H. El-Mallawany, Tellurite glass handbook: physical properties and data, CRC Press, Boca Raton, CA, 2002.
2. R. A. H. El-Mallawany, Tellurite glasses handbook: physical properties and data, CRC Press, Boca Raton, CA, 2014.
3. A. Jha, B. D. O. Richards, G. Jose, T. Toney Fernandez, C. J. Hill, J. Lousteau and P. Joshi, Int. Mater. Rev., 2012, 57, 357–382.
4. M. Venkateswarlu, S. Mahamuda, K. Swapna and M. Prasad, Opt. Mater., 2015, 39, 8–15.
5. E. S. Yousef, J. Alloys Compd., 2013, 561, 234–240.
6. Y. Benmadani, A. Kermaoui, M. Chalal, W. Khemici, A. Kellou and F. Pelle, Opt. Mater., 2013, 35, 2234–2240.
7. D. Sushama and P. Predeep, Int. J. Appl. Phys. Math., 2014, 4, 139–143.
8. B. Burtan, Z. Mazurak, J. Cisowski, M. Czaja, M. Lisiecki, W. Ryba-Romanowski, M. Reben and J. Wasylak, Opt. Mater., 2012, 34, 2050–2054.
9. E. P. Golis, M. Reben, J. Wasylak and J. Filipecki, Opt. Appl., 2008, 38, 163–168.
10. I. Jlassi, H. Elhouichet, M. Ferid, R. Chtourou and M. Oueslati, Opt. Mater., 2010, 32, 743–747.
11. H. Gebavi, D. Milanese, G. Liao, Q. Chen, M. Ferrari, M. Ivanda, O. Gamulin and S. Taccheo, J. Non-Cryst. Solids, 2009, 355, 548–555.
12. X. Shen, Q. Nie, T. Xu, S. Dai and X. Wang, J. Lumin., 2010, 130, 1353–1356.
13. Y. Benmadani, A. Kermaoui, M. Chalal, W. Khemici, A. Kellou and F. Pelle, Opt. Mater., 2013, 35, 2234–2240.
14. I. S. Yahia, K. A. Aly, Y. B. Saddeek, W. Dobrowolski, M. Arciszewska and L. Kilarski, J. Non-Cryst. Solids, 2013, 375, 69–73.
15. Y. Cheng, H. Xiao and W. Guo, Ceram. Int., 2008, 34, 1335–1339.
16. Y. B. Saddeek, I. S. Yahia, W. Dobrowolski, L. Kilanski, N. Romčević and M. Arciszewska, Optoelectron. Adv. Mater., Rapid Commun., 2009, 3, 559–564.
17. M. Reben, E. Golis, J. Filipecki, M. Sitarz, K. Kotynia, P. Jelen and I. Grelowska, Spectrochim. Acta, Part A, 2014, 129, 643–648.
18. J. Kansy, Nucl. Instrum. Methods Phys. Res., 1996, 374, 235–244.
19. J. Filipecki, E. Golis, M. Reben, K. Filipecka, A. Kocela and J. Wasylak, Optoelectron. Adv. Mater., Rapid Commun., 2013, 7, 1029–1031.
20. B. Burtan, M. Reben, J. Cisowski, J. Wasylak, N. Nosidlak, J. Jaglarz and B. Jarząbek, Acta Phys. Pol., A, 2011, 120, 579–581.
21. J. Liu, J. M. Cano-Torres, F. Esteban-Betegon, M. D. Serrano, C. Cascales, C. Zaldo, M. Rico, V. Griebner and V. Petrov, Opt. Laser Technol., 2007, 39, 558–561.
22. E. P. Golis and A. Ingram, J. Phys.: Conf. Ser., 2007, 79, 012003.
23. D. Munoz-Martín, M. A. Villegas, J. Gonzalo and J. M. Fernández-Navarro, J. Eur. Ceram. Soc., 2009, 29, 2903–2913.
24. G. Upender, V. G. Sathe and V. Chandra Mouli, Phys. B, 2010, 405, 1269–1273.
25. G. Upender and V. Chandra Mouli, J. Mol. Struct., 2011, 1006, 159–165.
26. A. Kocela, J. Filipecki, P. Korzekwa and E. Golis, Polym. Med., 2012, 42, 61–68.
27. R. Krause-Rehberg and H. S. Leipner, Defect Studies, Positron Annihilation in Semiconductors, Springer, Berlin, Heidelberg, New York, 1999.
28. P. K. Ojha, S. K. Rath, S. K. Sharma, K. Sudarshan, P. K. Pujari, T. K. Chongdar and N. M. Gokhale, Journal of Power Sources, 2015, 273, 937–944.
29. K. V. Krishnaiah, J. Marques-Hueso, K. Suresh, G. Venkataiah, B. S. Richards and C. K. Jayasankar, J. Lumin., 2016, 169, 270–276.
30. I. Coroiu, E. Culea and A. Darabont, J. Magn. Magn. Mater., 2005, 290–291, 997–1000.
31. E. Culea, L. Pop, S. Simon and M. Culea, J. Magn. Magn. Mater., 2005, 290–291, 1465–1468.
32. D. Souri, M. Mohammadi and H. Zaliani, Electron. Mater. Lett., 2014, 10, 1103–1108.
33. W. Li, K. Zou, M. Lu, B. Peng and W. Zhao, Int. J. Appl. Ceram. Technol., 2010, 7, 369–374.

---