Stabilization of Eu³⁺ under a reductive atmosphere by the Al³⁺ co-doping of Sr₂SiO₄:Eu²⁺/Eu³⁺

Abstract

Two series of materials based on a strontium orthosilicate matrix doped with europium and different concentrations of aluminum ions were obtained using different solid state synthesis strategies. The phase composition of the samples was investigated using the X-ray diffraction (XRD) technique. The results of the optical characterization indicated that the incorporation of aluminum ions causes the stabilization of the Eu³⁺ ions under reductive atmosphere. The concentration ratio of europium ions incorporated into the matrix in both oxidation states ([Eu³⁺]/[Eu²⁺]) can be controlled by changing the Al³⁺ concentration. The observed effect is interesting from the point of view of the design of phosphors used to obtain white light emitting diodes (WLEDs). The influence of temperature on the relative intensity of Eu²⁺ and Eu³⁺ emission bands was studied to investigate the range of the impact of the created compensation defects. It is shown that the Eu³⁺ to Eu²⁺ reduction process in Sr₂SiO₄ takes place due to the migration of the strontium vacancies to the surface.

---

Introduction

The stability of lanthanide ions in a given oxidation state, such as ytterbium, europium, samarium or thulium, introduced into the inorganic matrix has been investigated by many authors. However, europium ions have been studied most extensively.^1–9 This is due to the highly efficient emission observed in both the 2+ and 3+ oxidation states as well as the optimal location of the impurity levels relative to the valence and conduction bands of the matrix. In most cases, researchers have focused on the “abnormal reduction” of Eu³⁺ to Eu²⁺ when samples had been prepared in air at high temperatures and on the stability of europium ions when they exist only in the 2+ oxidation state. According to Su and co-workers^6,7 this type of reduction in solid state compounds occurs under specific conditions such as: (1) no oxidizing ions are present in the host compounds; (2) the dopant (the trivalent Eu³⁺ ion) replaces a divalent cation in the host; (3) the substituted cation has a similar radius to the divalent Eu²⁺ ion and (4) the host compound has an appropriate structure, consisting of tetrahedral groups (SO₄²⁻, PO₄³⁻, SiO₄⁴⁻, BO₄⁵⁻ or AlO₄⁵⁻). Additionally, the structure of the matrix has to be rigid and the europium ions substituting the host cations must be closely surrounded by the tetrahedral species. This can effectively shield Eu²⁺ ions from attack by oxygen. The “abnormal reduction” phenomenon has been found in many compounds of different classes. These are phosphates (Ba₃(PO₄)₂:Eu),¹ sulphates (BaSO₄:Eu),⁸ borates (SrB₄O₇:Eu),⁶ aluminates (Sr₄Al₁₄O₂₅:Eu)⁹ and silicates (BaMgSiO₄:Eu).⁷

There are significantly fewer reports concerning Eu³⁺ stabilization under the typical conditions of reduction (1200–1300 °C and 5–10% H₂ in N₂ or Ar) in the literature. Song et al. collected information about the oxidic compounds of strontium doped with Eu³⁺, which is stable under a slightly reductive atmosphere.⁴ The stability of europium was related to the Bond Valence Sum (BVS), which was derived from the second Pauling rule. The BVS was calculated for each strontium site in the given compound and related to the Global Instability Index (GII) derived by the authors. In the analysed compounds of strontium, the study showed that Eu³⁺ is stable when the BVS is greater than or equal to 2.0 and the GII does not exceed the so-called instability threshold line designated for particular classes of compounds (aluminates, borates, silicates, phosphates etc.).⁴

Work concerning the stabilization of lanthanides in the appropriate oxidation state can have a huge impact on the design of phosphors for white light emitting diodes (WLED). This is especially relevant to europium doped materials since it is possible to obtain materials doped simultaneously with Eu²⁺ and Eu³⁺. There is a lack of studies that raise the problem of the stability of lanthanide ions introduced into the inorganic matrix in different oxidation states. Therefore, it is very desirable to develop systematic investigations of systems containing Lnⁿ⁺ and Ln⁽ⁿ⁺¹⁾⁺ ions. The first fundamental studies on a system consisting of the matrix Lnⁿ⁺/Ln⁽ⁿ⁺¹⁾⁺ concerned the spectral properties of calcium silicate¹⁰ and lithium magnesium phosphate¹¹ doped with europium ions. In order to explain the possibility of introducing europium ions into the matrix in two oxidation states, the authors considered the influence of the value of the compensation defect creation energy and also, based on Dorenbos works,^12,13 the energy of the Fermi level on the stability of europium in a given oxidation state.

In this work, the role of co-doping with aluminium ions in the stabilization of Eu³⁺ under a reductive atmosphere is reported for the first time. Also a qualitative explanation of the stabilization phenomenon is proposed, considering the creation and elimination of both natural and chemically induced charge compensation defects.

Experimental details

Synthesis and structural characterization

The influence of the Al³⁺ co-dopant on the spectral properties of Sr₂SiO₄:Eu²⁺ was investigated using samples obtained via one- and two-step solid state synthesis strategies. In both cases, appropriate amounts of the reagents, such as strontium carbonate (Merck, opti pure), silica (Aldrich 99.999%), alumina (Aldrich 99.99%) and europium oxide (Aldrich 99.99%), were weighed to obtain the following molar composition: Sr_1.99Eu_0.01Si_1−xAl_xO₄, where x = 0.025, 0.05, 0.075 or 0.1. Then, they were thoroughly mixed for 1.5 h using a tempered steel planetary ball mill with a rotating speed of 400 rpm (Fritsh, Pulverisette 6) to obtain a homogenous distribution of substrates. In the next step, the obtained batches of reagents were calcined in different ways, depending on the type of synthesis strategy used. In the case of the two-step synthesis strategy, the series of four batches of reagents with different aluminium concentrations were firstly calcined at 1100 °C for 4 h in an inert gas atmosphere (Ar). Then, the obtained materials were cooled down, ground into a powder and once again calcined at 1250 °C for 4 h in a reducing atmosphere using a mixture of hydrogen (5 vol%) and nitrogen (95 vol%) in an electrical tubular furnace. In this way, two series of samples were obtained: series AbX – samples obtained before the reduction, doped with Eu³⁺ and series AaX – samples obtained after the reduction, doped with Eu²⁺ b – before and a – after the reduction and X – the aluminium concentration described as a mol percentage. The samples in the B series were obtained using a one-step strategy, where the reagents were calcined at 1250 °C for 4 h in a reducing atmosphere, using the aforementioned mixture of gases.

The quality and the purity of samples obtained after each step of the syntheses were examined with X-ray diffraction (XRD) methods, using a BRUKER D2PHASER system employing Cu K_α radiation and operated at 30 kV and 10 mA. The XRD patterns were collected using a scanning step of 0.02° and a counting time of 0.4 s per step. Semi-quantitative phase analysis was carried out using the DIFFRAC.EVA V4.1 evaluating application from BRUKER. The method consists of the comparison of the I/I_cor ratio (given in Powder Diffraction File card) of all the evaluated phases with their signal intensities.

Optical spectroscopy

Photoluminescence spectra were measured using a Shamrock SR750 D1 grating spectrometer equipped with an iDus 420 CCD detector (Andor Technology). An IK5352R-D He–Cd laser (Kimmon Koha) was used as an excitation source. To obtain luminescence spectra at different temperatures, the sample was placed in a closed-cycle type DE-204SL (Cryogenics Inc.) helium cryostat equipped with a type 336 (Lake Shore) temperature controller. All spectra were corrected with respect to the instrumental response.

Results and discussion

XRD measurements were carried out to study the phase composition and the influence of Al³⁺ on the structure of the obtained samples. Fig. 1a and b show the XRD patterns of the materials obtained after the first and the second step of the two-step synthesis strategy (series A), respectively. All the materials in the A series were a mixture of different crystallographic phases. The products of the synthesis collected before the reduction process (series Ab), besides the desired phase of strontium orthosilicate (PDF 00-039-1256), also contained strontium hydroxide (PDF 01-072-0057) as well as a small amount of silica (PDF 00-073-3470) as impurity phases. The compositions of the samples are presented in Table 1. After the reduction process, the composition of the materials (series Aa) slightly changed. The Sr₃SiO₅ (PDF 01-077-5979) phase appeared in place of SiO₂, whereas Sr(OH)₂ was still observed. Since Sr₃SiO₅ cannot be obtained under the applied synthesis conditions^14–18 it seems that the aluminium ions act as stabilizers of strontium oxosilicate. In the case of series B (Fig. 1c), the samples contained additional impurity phases in addition to strontium orthosilicate (PDF 00-039-1256). In contrast to the samples of series A, in this case, no other silicate phases were observed. Here, the presence of strontium hydroxide (PDF 00-027-0847) and strontium oxide (PDF 00-048-1477) in the product was confirmed. These are typical impurity phases remaining after the synthesis of strontium silicates.^19,20 The absence of the oxosilicate phase in the products suggests that the role of aluminium in the stabilization of oxosilicates is also related to the synthesis strategy.

Fig. 1 (a) X-ray diffraction patterns of the AbX series of Sr₂SiO₄:1% Eu, X% Al and standards. (b) X-ray diffraction patterns of the AaX series of Sr₂SiO₄:1% Eu, X% Al, and standards. (c) X-ray diffraction patterns of the BX series of Sr₂SiO₄:1% Eu, X% Al, and standards.

Table 1 Products of the syntheses and phase identification

Series	Materials	Al^3+ concentration [mol%]	Phase composition
A	Ab2.5	2.5	Sr2SiO4 89%; Sr(OH)2 8%; SiO2 3%
	Ab5.0	5.0	Sr2SiO4 83%; Sr(OH)2 11%; SiO2 6%
	Ab7.5	7.5	Sr2SiO4 87%; Sr(OH)2 7%; SiO2 6%
	Ab10	10	Sr2SiO4 86%; Sr(OH)2 9%; SiO2 5%
	Aa2.5	2.5	Sr2SiO4 87%; Sr(OH)2 9%; Sr3SiO5 4%
	Aa5.0	5.0	Sr2SiO4 89%; Sr(OH)2 6%; Sr3SiO5 5%
	Aa7.5	7.5	Sr2SiO4 84%; Sr(OH)2 8%; Sr3SiO5 8%
	Aa10	10	Sr2SiO4 85%; Sr(OH)2 8%; Sr3SiO5 7%
B	B2.5	2.5	Sr2SiO4 96%; Sr(OH)2 4%
	B5.0	5.0	Sr2SiO4 95%; Sr(OH)2 3%; SrO 2%
	B7.5	7.5	Sr2SiO4 93%; Sr(OH)2 5%; SrO 2%
	B10	10	Sr2SiO4 93%; Sr(OH)2 5%; SrO 2%

---

The europium ions incorporated into the inorganic matrix can exist in two stable oxidation states: Eu²⁺ and Eu³⁺. When the two valence electrons of the europium atom are transferred to the surrounding ligands, the ground state of the Eu²⁺ ion belongs to the electronic configuration 4f⁷. The spectroscopic properties of Eu²⁺, especially the luminescence, depend on the energy of the lowest state of the excited electronic configuration 4f⁶5d¹. The energies of the states from the 4f⁶5d¹ electronic configuration of the Eu²⁺ ions strongly depend on the host ligands due to spatial extension of the d orbitals. As a result, a broad, intensive emission band of Eu²⁺ can be observed in different regions of the spectrum depending on the matrix. Eu³⁺ is created when the three valence electrons are transferred to the surrounding ligands. The corresponding luminescence spectrum consists of sharp lines attributed to the intra-configurational f–f transitions inside the 4f⁶ electronic configuration. Their energies depend only very weakly on the host lattice, whereas the relative intensities of the particular bands depend on the local symmetry of the Eu³⁺ sites. The photoluminescence spectra of Sr_1.99Eu_0.01Si_1−xAl_xO₄ for different values of x, obtained after the calcination in inert gas atmosphere (the first step of the two-step synthesis – series AbX), are presented in Fig. 2. It can be seen that all the spectra consist of narrow lines located in the orange-red region that decay on a ms time scale. They are attributed to the ⁵D₀ → ⁷F_J transitions in Eu³⁺. The existence of Eu³⁺ ions occupying the divalent metal sites is typical for europium doped materials synthesized in an oxidative or inert gas atmosphere.^21–23 Additionally, it is observed that the intensity of the Eu³⁺ emission increases with increasing aluminium content. The relation between the emission intensity value and the Al³⁺ concentration is shown in the inset of Fig. 2. It can be seen that the integrated emission intensity is proportional to the aluminium content. An enhancement of Eu³⁺ emission is usually observed in inorganic matrices as a result of co-doping with monovalent ions that substitute divalent matrix cations.^23,24 The enhancement of Eu³⁺ emission by Al³⁺ ions occupying the Si⁴⁺ sites in Sr₂SiO₄ also has the same origin. This phenomenon has been explained by the compensation of the charge imbalance created after the aliovalent substitution of Me²⁺ by the Eu³⁺ ions. This decreases the number of defects that can quench the luminescence. The linear dependence of the Eu³⁺ emission intensity on the Al³⁺ content shown in Fig. 2 indicates that in our case the mechanism of the enhancement could be even simpler. Al³⁺ ions in the sites of Si⁴⁺ create additional centers of charge compensation induced by the introduction of Eu³⁺ in the sites of Sr²⁺. As a result, the number of Eu³⁺ incorporated in the lattice is increased. A qualitative comparison of the Eu³⁺ emission between the co-doped and unco-doped samples is shown in Fig. 3. It can be seen that the emission related to Eu³⁺ in the sample co-doped with aluminium is different to the emission of the reference sample. In the reference sample, the dominating emission line peaks are at 575 nm, whereas in the co-doped sample the bands located at longer wavelengths are much stronger. Additionally, the positions of some peaks are shifted. Comparison of these two spectra shows that the point symmetry of the Eu³⁺ site in the reference sample and in the co-doped sample are different. This can be explained by the effect of the creation of additional compensation centres by Al³⁺ in the Si⁴⁺ site.

Fig. 2 The emission spectra of the AbX series of Sr₂SiO₄:1% Eu, X% Al, collected at room temperature and λ_exc = 325 nm. The inset presents the values of the integrated emission intensity versus concentration of aluminium(III). The excitation wavelength corresponds to a maximum of the charge transfer (CT) transition in the excitation spectrum of Sr₂SiO₄:Eu³⁺.²⁷

Fig. 3 The emission spectra of Ab2.5 and reference sample collected at room temperature and λ_exc = 325 nm. The reference sample was Sr₂SiO₄:1% Eu³⁺ obtained using the solid state synthesis method described in detail in ref. 19.

During calcination under an H₂/N₂ atmosphere, the Eu³⁺ introduced into the Sr₂SiO₄ can be easily reduced to the form of Eu²⁺. This transformation results in a significant change in the luminescence spectrum. The sharp lines, decaying on a ms time scale, characteristic for transitions inside the 4f⁶ electronic configuration of Eu³⁺, are replaced by broad band luminescence, related to the parity allowed transition from the lowest state of the 4f⁶5d electronic configuration and the ground state of Eu²⁺. Due to the presence of the two different Sr²⁺ sites available for europium substitution, the emission spectrum of Sr₂SiO₄:Eu²⁺ consists of two strong and broad emission bands, which decay on a μs time scale. One of these corresponds to the 4f⁶5d–4f⁷ transitions located at blue-green (maximum at about 490 nm) and is attributed to the Eu²⁺ ions occupying the tenfold coordinated Sr²⁺ site. The other is located in the yellow-orange region (maximum at about 570 nm) and attributed to Eu²⁺ occupying the ninefold coordinated Sr²⁺ site in the lattice.²⁵ The relative intensities of the bands can be easily controlled by the excitation wavelength. When the material is excited with 325 nm, the emission band with a maximum at 490 nm strongly dominates.

In the case of the materials of the AaX series, which are obtained after the reduction process, a different situation is observed. The photoluminescence spectra of Sr₂SiO₄:Eu co-doped with aluminium, after the calcination, are presented in Fig. 4. As mentioned above, in the case of Sr₂SiO₄:Eu, calcination carried out under an H₂/N₂ atmosphere reduces Eu³⁺ to Eu²⁺ completely, thus no trace of luminescence related to Eu³⁺ is observed.²⁶ However, the emission spectra of all co-doped materials excited at 325 nm show the narrow lines attributed to the f–f transitions in Eu³⁺ besides the typical broad band with a maximum at 490 nm related to Eu²⁺. The existence of Eu³⁺ luminescence in the reduced samples is related to the fact that not all Eu³⁺ ions have been reduced to Eu²⁺. It can be seen that the intensity of the Eu³⁺ emission with respect to the Eu²⁺ emission increases with increasing Al³⁺ concentration. It can be assumed that the intensity of the Eu³⁺ emission is proportional to the amount of europium ions remaining in the trivalent state. Fig. 4 presents the emission spectra normalized with respect to the maximum intensity of the Eu²⁺ emission band located at 490 nm. It can be seen that in the materials co-doped with 2.5 and 5.0 mol% of Al³⁺, the intensity of the Eu³⁺ emission are similar. It is also much weaker than in the materials co-doped with 7.5 and 10 mol% of Al³⁺. Following the assumption that the intensity of luminescence is proportional to the number of ions, one notices that the number of unreduced Eu³⁺ ions increases significantly when the Al³⁺ ion concentration changes from 5.0 to 7.5 mol%.

Fig. 4 The emission spectra of the AaX series of Sr₂SiO₄:1% Eu, X% Al, collected at room temperature and λ_exc = 325 nm.

In the case of the materials obtained from the one-step synthesis strategy, where the calcination and reduction processes are carried out simultaneously, this phenomenon is also observed. Fig. 5 shows the emission spectra excited at 325 nm and collected for respective materials in series B. Here, the influence of the Al³⁺ ions on the intensity of the Eu³⁺ emission bands is more specific in comparison with the materials of the AaX series. The differences between the Eu³⁺ emission intensity of materials co-doped with 2.5 and 5.0 mol% as well as 7.5 and 10 mol% of Al³⁺ can be easily observed. However, the relationship between the Al³⁺ concentration and the Eu³⁺ emission band intensity is still not linear. The emission spectra excited at 442 nm are presented in Fig. 6. Under this excitation, only the emission of Eu²⁺ ions shows broad band luminescence, which is related to the transitions in Eu²⁺ in the ninefold coordinated sites. No emission from Eu³⁺ can be observed. It is apparent that the emission intensity decreases about five times with increasing Al³⁺ concentration (see the inset in Fig. 6, where the relative integrated intensity versus aluminium content is presented). These results show that co-doping with Al³⁺ diminishes the amount of Eu²⁺ (in the ninefold coordinated sites) approximately proportionally to the Al content. There is no evidence to show whether (and how) the Al influences the tenfold coordinated Eu sites. This problem requires additional investigation. However, since the intensity of the Eu³⁺ emission increases in a different way, probably not all the Eu³⁺ sites compensated by Al³⁺ contribute to the emission.

Fig. 5 The emission spectra of the BX series of Sr₂SiO₄:1% Eu, X% Al, collected at room temperature and λ_exc = 325 nm.

Fig. 6 The emission spectra of the BX series of Sr₂SiO₄:1% Eu, X% Al, collected at room temperature and λ_exc = 425 nm.

The results described above show that the Al³⁺ co-dopant can stabilize Eu³⁺ under reduction conditions in both cases. However, the absence of a linear relationship between Al³⁺ concentration and Eu³⁺ emission band intensity after the reduction process requires additional investigation. It is worth noting that co-doping can influence the Eu²⁺/Eu³⁺ ratio by changing the Al³⁺ concentration if the parameters of synthesis are well optimized.

In a typical synthesis of materials doped with lanthanide ions introduced in both oxidation states, the only parameter influencing the concentration ratio of lanthanide ions on the appropriate oxidation state is the heating time of the material under a suitable atmosphere. However, control of the concentration ratio by co-doping can be much more effective. The observed stabilization process is promising from the point of view of phosphor design used to obtain white light emitting diodes (WLEDs). Fig. 7 represents the Commission Internationale de l'Eclairage chromaticity diagrams of the materials of series AaX and BX obtained after reduction. The results show that the investigated phenomenon enables tunable emission color with the use of only one rear earth element and a cheap co-dopant at a fixed excitation wavelength.

Fig. 7 CIE coordination diagrams of the samples series (AaX – on top and BX – bottom) obtained after the reduction process.

Taking into account the criteria for the existence of Eu²⁺ and Eu³⁺ in a given matrix as well as the influence of compensation defects on the Fermi level, as described in the ref. 10 and 12, the influence of temperature on the relative intensity of the Eu²⁺ and Eu³⁺ emission bands was investigated. In Fig. 8, the emission spectra of the sample B7.5 measured at different temperatures (from 10 up to 375 K) are presented. The excitation wavelength of 325 nm was fixed during the experiment since the different excitation energies influence the intensity of the Eu²⁺ ions emission. As can be seen, the relative intensity of the Eu²⁺ and Eu³⁺ emission bands does not depend on temperature. This result suggests that the charge compensation defects, which are responsible for the stabilization of Eu³⁺ ions introduced into the Sr²⁺ sites, have a local character and a long range Coulomb potential is not created. A similar situation is described in detail in ref. 10 and concerns Ca₂SiO₄ doped simultaneously with Eu²⁺ and Eu³⁺.

Fig. 8 The emission spectra of the B7.5 sample (Sr₂SiO₄:1% Eu, 7.5% Al) collected at different temperatures (10–375 K) and λ_exc = 325 nm.

The phenomenon of Eu³⁺ stabilization by Al³⁺ co-doping under reduction conditions can be qualitatively explained considering the creation and elimination of natural and chemically induced charge compensation defects. First of all, to understand the stabilization of Eu³⁺ by co-doping with Al³⁺, the different mechanisms of compensation of the Eu³⁺ occupying divalent cation site should be considered. In the absence of aluminium, the positive charge of Eu³⁺ in the Sr²⁺ site can be compensated by Sr²⁺ vacancies, , which are created by the excess of oxygen during the synthesis in ambient atmosphere. creates an acceptor state that binds one or two electrons and diminishes the Fermi level below the energy of the ground state of Eu²⁺. In such a case, the highest occupied state of europium is the ground state of Eu³⁺ (see Fig. 9d). This type of compensation defect can be considered as natural. A single Sr²⁺ vacancy can compensate one or two Eu³⁺ ions (see Fig. 9a). During the reduction process, H₂ does not penetrate the material deeply and reacts with the oxygen ions located at the surface of the matrix. The product of this reaction, H₂O, evaporates leaving the oxygen vacancies. This is equivalent to the excess of Sr²⁺ ions on the surface. Further, the Sr²⁺ migrates from the surface to the bulk crystal. This process can also be considered as migration of the to the surface. Finally, when all the are filled with Sr²⁺, all the Eu³⁺ ions can be reduced to Eu²⁺. This migration is shown in Fig. 9b, and the final state is shown in Fig. 9c.

Fig. 9 A scheme of the elimination of natural compensation defects (a–c) and energetic diagrams (d and e), which describe the situation (a and c), respectively.

The introduction of Al³⁺ in the Si⁴⁺ site creates a chemically induced defect that can also create an acceptor level, which can capture one electron. The single Al³⁺ ion can compensate only one Eu³⁺ ion in the strontium site. The situation before the reduction is presented in Fig. 9d. However, when the system is co-doped with aluminium, even though the reduction atmosphere creates oxygen vacancies, it may not cause the migration of Al³⁺ ions. Thus, the Eu³⁺ ions that were compensated by Al³⁺ in the Si⁴⁺ site cannot be reduced and remain in the Eu³⁺ state. The situation is presented in Fig. 9e.

The energy necessary for migration of is smaller than the energy for migration of the oxygen vacancy, , due to the difference in the type of strontium and oxygen bonding with the nearest neighbours. In the orthosilicate matrix, the oxygen ions are a part of a siliceous tetrahedron, in which elements are bonded with partially covalent bonds. It should be emphasized that are responsible for compensation of Eu³⁺ and their migration to the surface is responsible for the reduction of Eu³⁺ to Eu²⁺. If reduction of Eu³⁺ to Eu²⁺ took place due to the migration of from the surface to the bulk crystal, the compensation effect of Al³⁺ replacing Si⁴⁺ would be cancelled by the appearance of in the bulk crystal. Our research has shown that this is not the case.

Conclusions

The natural valence state of europium ions (+3) introduced into the strontium orthosilicate matrix can be easily reduced to +2 when the synthesis of the material is carried out under a reductive atmosphere. In the case described in this contribution, the reduction process is not realized for the part of Eu³⁺ ions due to the co-doping of the phosphor with aluminium ions. To the best of our knowledge, the control of the Eu²⁺/Eu³⁺ ratio by the Al co-dopant is reported here for the first time. The Eu³⁺ stabilization can be qualitatively explained by the differences in the energy necessary for the migration of the studied compensation defects (, and ). Our research shows that the Eu³⁺ to Eu²⁺ reduction process in Sr₂SiO₄ takes place due to the migration of the strontium vacancies to the surface.

Acknowledgements

The work has been partially supported by the National Science Centre, Poland, agreement number: UMO 2014/13/D/ST3/04032 and POIG.01.01.02-02-006/09 project co-funded by European Regional Development Fund within the Innovative Economy Program. Priority I, Activity 1.1. Sub-activity 1.1.2.

Notes and references

1. I. Tale, P. Kulis and V. Kronghauz, J. Lumin., 1979, 20, 343.
2. X. M. Zhang, F. G. Meng, W. L. Li and H. J. Seo, Ceram. Int., 2013, 39, 8975.
3. Z. Pei, Q. Su and J. Zhang, J. Alloys Compd., 1993, 198, 51.
4. Z. Song, J. Liao, X. Ding, Q. L. Liu and T. Zhou, J. Lumin., 2012, 132, 1768.
5. Q. Su, H. Liang, T. Hu, Y. Tao and T. Liu, J. Alloys Compd., 2002, 344, 132.
6. Z. Pei, Q. Zheng and Q. Su, J. Phys. Chem. Solids, 2000, 61, 9.
7. M. Peng, Z. Pei, G. Hong and Q. Su, J. Mater. Chem., 2003, 13, 1202.
8. U. Madhusoodanan, M. T. Jose and A. R. Lakshmanan, Radiat. Meas., 1999, 30, 65.
9. M. Y. Peng, Z. W. Pei, G. Y. Hong and Q. Su, Chem. Phys. Lett., 2003, 371, 1.
10. A. Baran, J. Barzowska, M. Grinberg, S. Mahlik, K. Szczodrowski and Y. Zorenko, Opt. Mater., 2013, 35, 2107.
11. A. Baran, S. Mahlik, M. Grinberg, P. Cai, S. I. Kim and H. J. Seo, J. Phys.: Condens. Matter, 2014, 26, 385401.
12. P. Dorenbos, J. Mater. Chem., 2012, 22, 22344.
13. P. Dorenbos, Chem. Mater., 2005, 17, 6452.
14. L. Chen, A. Luo, Y. Jiang, F. Liu, X. Deng, S. Xue, X. Chen and Y. Zhang, Mater. Lett., 2013, 106, 428.
15. C. Shen, Y. Yang, S. Jin, J. Ming, H. Feng and Z. Xu, Optik, 2010, 121, 1487.
16. Z. Wang, B. Yang, P. Li, Z. Yang and Q. Guo, Phys. B, 2012, 407, 1282.
17. C. Guang, L. Quansheng, C. Liqun, L. Liping, S. Haiying, W. Yiqing, B. Zhaohui, Z. Xiyan and Q. Guanming, J. Rare Earths, 2010, 28, 526.
18. W. Xiaochun, Z. Xiyan, W. Chen, Q. Weida and S. Jiaxun, J. Rare Earths, 2013, 31, 456.
19. K. Szczodrowski, A. Chruścińska, J. Barzowska, R. Przegiętka, K. Anders, R. Piramidowicz and M. Grinberg, RSC Adv., 2015, 5, 65236.
20. H. P. Jung, A. Wonsik and J. K. Young, Ceram. Interfaces, 2015, 41, S734.
21. Y. Qiao, X. Zhang, X. Ye, Y. Chen and H. Guo, J. Rare Earths, 2009, 27, 323.
22. R.-Y. Yang, H.-Y. Chen, S.-J. Chang and Y.-K. Yang, J. Lumin., 2012, 132, 780.
23. H. Nagabhushana, D. V. Sunitha, S. C. Sharma, B. Daruka Prasad, B. M. Nagabhushana and R. P. S. Chakradhar, J. Alloys Compd., 2014, 595, 192.
24. H. Luo, J. Liu, X. Zheng, L. Han, K. Ren and X. Yu, J. Mater. Chem., 2012, 22, 15887.
25. J. Barzowska, K. Szczodrowski, M. Krośnicki, B. Kukliński and M. Grinberg, Opt. Mater., 2012, 34, 2095.
26. J. H. Lee and Y. J. Kim, Mater. Sci. Eng., B, 2008, 146, 99.
27. A. Nag and T. R. N. Kutty, J. Mater. Chem., 2004, 14, 1598.

---