Stoichiometric Glass Research Articles

Crystal growth in oxide melts—From CALPHAD thermodynamic modeling to statistical prediction

Using a thermodynamic database approach (CALPHAD), thermodynamic parameters of crystal growth of 31 stoichiometric and non-stoichiometric glasses are calculated. Together with these parameters, the measured growth rates, taken from both literature and in-house measurements, are interpreted in the framework of four interface-controlled growth models: normal, screw-dislocation, 2D growth and a combination of the latter two. A multiple linear regression model is used to explain the maximum crystal growth rate and viscosity at the liquidus temperature with the above growth mechanisms. This finding seems to reflect a general trend and can therefore be used to avoid unintentional devitrification phenomena or to derive their temperature-time range based on the sole knowledge of glass viscosity and liquidus temperature. In future research, it can also help to reduce the experimental workload required for the development of novel glasses and glass-ceramics.

Atomic and electronic origin of robust off-state insulation properties in Al-rich AlxTey glass for ovonic threshold switching applications

Ovonic threshold switching (OTS) selectors play a critical role in suppressing the sneak-path current of three-dimensional crossbar integration circuits. Compared to conventional nonmetal-telluride OTS selectors, selectors based on AlxTey glass are found to have both satisfactory on-state current and selectivity. However, it is unclear why the Al-rich AlxTey glass-based OTS selectors have robust insulation properties for reducing the off-state current. This work reveals the structure–property correlations of amorphous AlxTey at the atomic scale by first-principles calculations. It is found that the stoichiometric Al2Te3 glass tends to have a clean bandgap owing to the covalent and dative bonds formed by non-equivalent sp3-hybridized Al orbitals and the lone-pair electrons of Te. Unexpectedly, for Al-rich AlxTey glass (Al2.21Te2.79), the Al–Al bonds formed by redundant Al-atoms have an integrated crystal orbital bond index (ICOBI) of 0.8–0.9, which is much larger than that of Al–Al bonds in pure metals (0.227), indicating they are covalent. It is the covalent Al–Al bonds that ensure the robust insulation characteristics of Al-rich AlxTey glass, while the Te–Te interaction in the Al-poor AlxTey glass (Al1.79Te3.21) produces midgap states, thereby reducing the insulativity. The presented atomic and electronic pictures here will provide useful theoretical insights for designing OTS selectors with improved performances.

Crystallization in Ga–Sb–Se glasses and influence of the Se content

Crystallization in glasses composed of Ga, Sb and Se has been studied as a function of Se content. Six glass compositions are selected (Ga8Sb27Se65, Ga12Sb23Se65, Ga8Sb32Se60, Ga10Sb30Se60, Ga8Sb37Se55 and Ga8Sb40Se52), with different Se contents. Glass-ceramics are obtained by heating the glass 5 °C above its glass transition temperature. Glasses and glass-ceramics are characterized by differential scanning calorimetry, Raman spectroscopy, infrared spectroscopy and X-ray diffraction. A correlation between the Se content, the nature of the phase crystallizing and the electrical conducting properties of the material is evidenced. On one hand, antimony selenide Sb2Se3 crystallizes in over-stoichiometric and stoichiometric glasses compared to the x Ga2Se3 -(100-x) Sb2Se3 line. On the other hand, an unknown phase crystallizes in sub-stoichiometric glasses, followed by Sb2Se3. The resulting glass-ceramics are characterized by a conductivity reaching up to 2 S·cm−1, two order of magnitude higher than other Se-based glass-ceramics.

A comprehensive approach to crystallization sequence in novel MgO/CaO-Al2O3-SiO2 glass-ceramics by Ostwald's step rule and composition fluctuation model

An understanding of the crystallization sequence (the formation order of crystalline phases) of glasses is crucial for the processing and composition design of novel glass-ceramics. A comprehensive approach to the crystallization sequence in novel MgO/CaO-Al2O3-SiO2 (MAS/CAS) glass-ceramics was performed from the Ostwald's step rule and the model of the distribution of nanoscale composition fluctuations (DNCF) for the first time. The stoichiometric cordierite Mg2Al4Si5O18 glass exhibits the crystallization sequence predicted by the Ostwald's step rule and a narrow DNCF model, i.e., metastable β-quartz solid solutions (β-QSSs) → high-temperature type indialite → low-temperature type α-cordierite. A broad DNCF model was proposed to present in osumilite Mg2Al4Si11O30 and pyrope Mg3Al2Si3O12 glasses, resulting in the initial formation of β-QSSs or forsterite 2MgO.SiO2 prior to α-cordierite formation. The crystallization sequence in stoichiometric anorthite CaAl2Si2O8 glass was also understood along the Ostwald's step rule and a narrow DNCF model. The importance of nanoscale composition fluctuations, in particular the formation of MgO/CaO-rich fragile regions, was emphasized in the crystallization of MAS and CAS-based glasses having strong Al-O-Si linkages between AlO4 and SiO4 tetrahedra. The present article will contribute to the comprehensive design of new innovative aluminosilicate-based glass-ceramics.

Compressibility, Metallization, and Relaxation in Nonstoichiometric Chalcogenide Glass g-As3Te2 at High Hydrostatic Pressure versus “Classic” g-As2Te3 Glass

The volume and conductivity of nonstoichiometric chalcogenide glass g-As3Te2 have been investigated at high hydrostatic pressures (up to 8.5 GPa), and results have been compared with earlier data for stoichiometric chalcogenide glass g-As2Te3. Structural and Raman studies of g-As3Te2 glass have revealed a greater significance of As–As pair correlations in the range of medium-range order compared with “classic” chalcogenide glass g-As2Te3. Even at such a large excess of arsenic, a high concentration of “improper” Te–Te neighbors has been observed because of chemical disorder. Under normal conditions, the thermal gap (0.43–0.48 eV) and resistivity (104 Ω cm) of glass g-As3Te2 are greater than those of g-As2Te3. The elastic behavior of g-As3Te2 glass, as well as of g-As2Te3, under compression has been observed at pressures up to 1 GPa, the initial values of bulk moduli for these glasses being nearly coincident. Polyamorphic transformation in g-As3Te2 (with softening of relaxing bulk modulus) is more diffuse and extends to higher pressures (from 1.5 to 4.0 GPa). The metallization process in g-As3Te2 is also more diffuse: metallic conductivity is reached at pressures of 5.5–6.0 GPa. As in the case of the stoichiometric glass, the baric dependences of the bulk modulus exhibit a kink in the pressure range 4–5 GPa. Up to maximal pressures, the volume and resistivity relax logarithmically in time with roughly the same rate as in the case of g-As2Te3. The residual densification of g-As3Te2 after pressure release is roughly twice as high as for g-As2Te3 and equals 3.5%, the conductivity of the compacted glass is about three orders of magnitude higher than that of the as-prepared sample. Under normal conditions, a considerable relaxation of the volume and resistivity has been observed. As for densified g-GeS2 glass, the logarithmic kinetics of this relaxation has been successfully described in terms of our earlier model based on the concept of relaxation self-organized criticality with the activation energy (1.3 eV) remaining unchanged up to 5 × 106 s.

Short range order and topology of binary Ge-S glasses

Short range order and topology of GexS100-x glasses over a broad composition range (20 ≤ x ≤ 42 in at%) was investigated by neutron diffraction, X-ray diffraction, and Ge K-edge extended X-ray absorption fine structure (EXAFS) measurements. The experimental data sets were fitted simultaneously in the framework of the reverse Monte Carlo simulation method. It was found that both constituents (Ge and S) satisfy the Mott-rule in all investigated glasses: Ge and S atoms have 4 and 2 neighbours, respectively. The structure of these glasses can be described with the chemically ordered network model: Ge-S bonds are preferred; S-S bonds are present only in S-rich glasses. Dedicated simulations showed that Ge-Ge bonds are necessary in Ge-rich glasses. Connections between Ge atoms (such as edge-sharing GeS4/2 tetrahedra) in stoichiometric and S-rich glasses were analysed. The frequency of primitive rings was also calculated.

Microstructural and residual stress effects on toughening of stoichiometric BaO.2SiO2 glass-ceramics

Barium disilicate (BaO.2SiO2=BS2) glass is one of the few stoichiometric glasses that nucleates internally, homogeneously via thermal treatment. This system has been scarcely assessed in microstructure-property studies. Here we address fracture strength and toughness (KIC) variation as a function of crystallized volume fraction and crystal size, as well as the possible effect of residual stresses (RS) in BS2 glass-ceramics (GCs) by independently varying these two microstructural parameters. KIC increased with spherulite size and crystallized volume fraction. KIC variation with crystallized volume fraction was similar for GCs with different crystal sizes. Combination of the current findings on BaO.2SiO2 (crystals under tensile RS) with previous studies of Li2O.2SiO2 GCs (compressive RS) indicates that crystallization of a tougher phase – not type of residual stress in the crystals – is the crucial parameter controlling fracture toughness and strength. These findings are quite useful to design novel strong and tough GCs.

The best diffusivity proxy for crystal nucleation in stoichiometric oxide glasses

Due to the difficulty in measuring the mobility of the “structural units” of supercooled liquids and glasses during crystal nucleation, the effective diffusivity (D) has been frequently estimated via viscosity (Dη) or nucleation time-lags (Dτ). However, it has been recently reported that, below the glass transition temperature (Tg), the experimental steady-state nucleation rates (Ist) can reach much higher values than those predicted by the Classical Nucleation Theory (CNT) when assuming D ≈ Dη or D ≈ Dτ. Hence, another possibility would be inferring D from experimental crystal growth velocity data (D ≈ DU), which is a natural approach since nucleation and growth are likely controlled by the same mass transfer process through the crystal/liquid interface. Therefore, the approximations D ≈ Dη and D ≈ DU were tested in this work using lithium disilicate glass as a model system. To avoid the influence of compositional changes, we measured the nucleation rates, growth velocity, and viscosity using samples of the same glass batch. With this strategy, we found that the very long experimental nucleation times implemented below Tg shifted the experimental temperature of maximum nucleation rate to temperatures lower than those previously reported for this glass former. Most important is that the CNT gives a much better description of the experimental Ist(T) data in the whole analyzed temperature range if D is approximated by DU instead of Dη, indicating that crystal growth is indeed an adequate proxy for nucleation diffusivity.

A new iron-phosphate compound (Fe7P11O38) obtained by pyrophosphate stoichiometric glass devitrification

Iron phosphates are a wide group of compounds that possess versatile applications. Their properties are strongly dependent on the role and position of iron in their structure. Iron, because of its chemical character, is able to easily change its redox state and accommodate different chemical surroundings. Thus, iron-phosphate crystallography is relatively complex. In addition, the compounds possess intriguing magnetic and electric properties. In this paper, we present crystal structure properties of a newly developed iron-phosphate compound that was obtained by devitrification from iron-phosphate glass of pyrophosphate stoichiometry. Based on X-ray diffraction (XRD) studies, the new compound (Fe7P11O38) was shown to adopt the hexagonal space group P63 (No. 173) in which iron is present as Fe3+ in two inequivalent octahedral and one tetrahedral positions. The results were confirmed by Raman and Mössbauer spectroscopies, and appropriate band positions, as well as hyperfine interaction parameters, are assigned and discussed. The magnetic and electric properties of the compound were predicted by ab initio simulations. It was observed that iron magnetic moments are coupled antiferromagnetically and that the total magnetic moment of the unit cell has an integer value of 2 µB. Electronic band structure calculations showed that the material has half-metallic properties.

Mechanical properties and band gap estimations of stoichiometric GeSe2-As2Se3 glasses

The elastic moduli (Bulk (Ke), Micro-hardness (He), Young (Ye)) and Poisson’s ratio (Pr) of the prepared xGeSe2-(100-x)As2Se3 (0 ≤ x ≤ 100) glasses are estimated using the ultrasonic velocities (Longitudinal (νl) and Transverse wave (νt)) and the density (ρ). The glass transition temperature (Tg) of the glasses has been obtained using the DSC thermograms recorded at a heating rate 10 K/min. As well, many physical and structural features have been obtained theoretically by applying the Chemical Bond Approach (CBA). Theoretical evaluations of optical band-gap (Eg) and the glass-transition temperature (Tg) for the glasses have been studied. It was found that the obtained theoretical values of the parameters Eg and Tg agree with the experimental values within uncertainty < 4%. Also, rigidity parameters such as the heat of atomization (Hs), the mean bond energy (Es), the cohesive energy (CE), and the overall average bond energy (< E > ) have been estimated. The dependence of Eg on the rigidity parameters (Hs, Es, CE, and < E > ) was verified and discussed. Furthermore, the coordination number (CN), lone pair electrons (LP), and crosslinking density (CD) were estimated. Eg values increased from 1.6 to 2.04 eV with increasing the GeSe2 concentration from 0 to 100 at%. So, these materials can be used as optical absorbers in solar cells and sensors.

Crystallization mechanisms of cordierite glass-ceramics with “surface-center” crystallization behavior

Cordierite glass-ceramics usually begin to crystallize from the surface. As an efficient nucleating agent, TiO2 can promote the rapid transformation of glass to bulk crystallization, but it is easy to cause the increase of dielectric constant and light absorption. High crystallinity cordierite glass-ceramics were prepared by optimizing the heat treatment process without or with different nucleating agents in stoichiometric cordierite glasses. The results show that the crystallization mechanisms of glasses without and with ZrO2+P2O5 as nucleation agents are controlled by surface crystallization. While, the glass with TiO2+ZrO2+P2O5 as nucleation agents have the tendency to be bulk crystallization. The studied glasses are crystallized from surface and have different crystallization orientations with the inner glass. The thickness of crystalline layer increased with the increasing of heating temperatures, but the “surface-center” crystallization process cannot complete by further increasing heating temperatures because of softening deformation of glasses. At 1020 ℃, the glasses complete the “surface-center” crystallization for long durations. The glasses without nucleation agents and with ZrO2+P2O5 require 10 h, but the glass with TiO2+ZrO2+P2O5 complete for 5 h. Although all the three glasses complete the “surface-center” crystallization, the glasses with nucleation agents show the higher crystallinity upon the same heat treatments. Finally, glass-ceramics with excellent performance were obtained, for example, the Z1# glass-ceramic have the high microhardness ∼7.4 GPa, low thermal expansion coefficient ∼1.4☓10−6 ℃−1 at 20–300 ℃, and relatively high thermal conductivity ∼2.4 W/mK. It also exhibits low dielectric constant and loss, which was ∼4.5 and ∼1.2☓10−3 at 1 MHz, ∼ 4.9 and 2.3☓10−3 at 10.5 GHz..

Further evidence against the alleged failure of the classical nucleation theory below the glass transition range

AbstractWe collected a plethora of new data to test the hypothesis that the failure of the Classical Nucleation Theory (CNT) below the glass transition range is just an experimental artifact. Since reaching the steady‐state nucleation regime takes a significant time for treatments below the glass transition temperature, data collected in this temperature range tend not to have reached a steady state. Because of this potential problem, we examined the CNT using new experimental data for three stoichiometric silicate glasses: Li2Si2O5, BaSi2O5, and Na4CaSi3O9. We also measured the equilibrium viscosity for the studied glass batches and used it as a proxy for the effective diffusion coefficient. The analysis was conducted by applying a steady‐state criterion and evaluating the error propagation throughout all calculations. Using this rigorous procedure, we have not observed the alleged CNT failure. Our comprehensive results support recent studies questioning this possible CNT failure helping solve a longstanding problem in glass science.

Insights into the physical aging in chalcogenide glasses: A case study of a first-generation As2Se3 binary glass

• The experimental/theoretical understanding of physical aging in chalcogenide glasses has been reviewed. • General symptoms, peculiarities, and characteristics of PhA have been discussed. • Investigations of PhA in industrial glass (As 2 Se 3 ) have been reported. The time-dependent variation in the physical properties is one of the most interesting and mysterious behaviors of melt-quenched disordered solids (like polymer glasses, metallic glasses, and especially in chalcogenide glasses). This phenomenon is recognized as physical aging which reveals that the as-quenched glass stays in a meta-stable state (i.e., a thermodynamically non-equilibrium state) and so a thermally activated structural relaxation is the general characteristics of each glass as a consequence of natural or artificial physical aging (PhA). This review starts with the general introduction of PhA and its features. Different perceptive and peculiarities of PhA have been discussed keeping in mind the case of long-term PhA and anti-aging effects of arsenic in the chalcogenide glasses (ChGs) of the Se-As system. Further, we have reported the case study of As 2 Se 3 stoichiometric glass and the results of some new observations during the natural aging of the Arsenic Tri-Selenide glass (g-As 2 Se 3 ). The thorough inspection of PhA effects in this industrial glass is done after storing at ambient temperature in a glovebox for ten years. Thermal analysis is done by using differential scanning calorimetry. The XRD and TEM/SEM techniques have been employed for structural analysis. The information of various kinds of bonding is acquired by using Raman spectroscopy. Also, we have determined the thermal stability parameter, Vickers hardness, compactness, fragility, and density of the fresh and old samples of As 2 Se 3 glass to see the consequences of PhA. Finally, the new advances and present status related to the studies of PhA have been discussed.

Decoupling Microstructural and Residual Stress Effects on Glass-Ceramic Toughening

Barium disilicate (BaO.2SiO 2 = BS2) is one of the very few stoichiometric glasses that allows for accurate control of the desired microstructure via thermal treatment, making it a (scarcely studied) model system for microstructure-property studies. Here, we performed a systematic research work on the variation in hardness, elastic modulus, fracture strength, and toughness as a function of the crystalline volumetric fraction, crystal size, and residual stresses in BS2 glass-ceramics (GC). These microstructural features were independently modified; the average crystal diameter varied from 5 to 100 μm and the crystallized volumetric fraction from 0 to 68%. The internal residual stresses in the crystals, are tensile in this system . Samples with an average spherulite size above 30 µm showed spontaneously fractured crystals due to the residual stresses. The samples with 5 and 10 µm spherulites have not cracked because the spherulites have a high volume fraction of residual glass (74%) and moderate internal stresses (40-70 GPa). The fracture toughness, K IC , increased with the spherulite size and volume fraction. However, the residual glass inside the spherulites rendered crack bowing, bridging, and trapping ineffective. The variation of K IC with the crystallized volume fraction is similar for GCs with different crystal sizes. Also, a comparison with a lithium silicate glass-ceramic showing compressive residual stresses yielded similar results. These combined findings indicate that the crystallization of a tougher phase is the crucial parameter controlling fracture toughness in these materials. The results with this model material can be extended to design novel strong and tough glass-ceramics.

Sound Speed in Glassy AsxSe1−x (x = 0.4, 0.5, and 0.6) by Inelastic X‐Ray Scattering

Inelastic X‐ray scattering (IXS) measurements are carried out for glassy , , and to investigate atomic dynamics in the glasses. Momentum transfer (Q) dependence of the acoustic excitation energy in each glass follows the ultrasonic sound speed. No positive dispersion is observed in these glasses although liquid near the melting point shows it. The stoichiometric glass shows a maximum sound speed and the speed decreases with increasing As concentration. The results seem to be correlated with a prediction by rigidity theory that glassy is most stable.

Which glass stability parameters can assess the glass‐forming ability of oxide systems?

AbstractGlass‐forming ability (GFA) is a property of utmost importance in glass science and technology. In this paper, we used a statistical methodology—involving bootstrap sampling and the Wilcoxon test—to find out which glass stability (GS) parameters best predict the GFA. The GS parameters are based on the characteristic temperatures: glass transition temperature (Tg), onset of crystallization (Tx), crystallization peak (Tc) and liquidus (Tl). We collected the necessary data for twelve stoichiometric oxide glasses using finely ground powders to favor heterogeneous nucleation (the most common case). We found that some GS parameters predict the GFA of these oxide glasses quite well, whereas others perform poorly. The Weinberg parameter Kw = (Tc − Tg)/Tl ranked first, closely followed by the Hrüby and the Lu‐Liu parameter. Our results corroborate previous reports carried out using a smaller number of glasses, much less GS parameters, and less rigorous statistics. We also found that using Tc instead of Tx improved the predictive power of these parameters. Finally, the Jezica = η (Tl)/Tl2 (η = viscosity) parameter also ranked reasonable well, and it is the only parameter considered here that predicts the GFA without requiring the production of a glass piece, ie, without relying on any crystallization information.

Crystallization behavior and IR structure of yttrium aluminosilicate glasses

The crystallization of four Y2O3-Al2O3-SiO2 (YAS) glasses were investigated to prepare YAS glass ceramics precipitated singly/mainly Y2Si2O7 or Y4.67(SiO4)3O apatite, and to explore the crystallization difference between the stoichiometric parent glass (SPG) and non-stoichiometric parent glass (NSPG). The DSC results revealed that glass locating at the higher liquidus surface temperature has lower crystallization peak temperature, which indicating that the corresponding glass has higher crystallization potential to crystallize easily. Crystallization of the NSPG samples is along surface and caused by phase separation, while SPG sample is the surface crystallization at the first exothermic peak temperature and overall crystallization at the second exothermic peak temperature. Glass ceramics only containing y-Y2Si2O7 or Y4.67(SiO4)3O apatite are obtained successfully, and which are illustrated by fitting FTIR spectra. These results can provide technical guide for controlling the crystallization process and the types of precipitated crystals in YAS glass for different application potentials.

Effect of non-stoichiometry on the crystal nucleation and growth in oxide glasses

Non-stoichiometric glasses (NSG) are much more common than stoichiometric compositions. However, due to inherent difficulties, fundamental studies of crystallization kinetics of NSG are much less frequent. To shed light on the crystal nucleation and growth kinetics of NSG, we adopted a nucleation kinetics model, leaving the interfacial energy and diffusion coefficient as free parameters, to explain experimental nucleation data of glasses of three compositions in the pseudo-binary Li2O·2SiO2–BaO·2SiO2 model system. We show that, as the glass composition approaches the eutectic, the nucleation rates drop drastically, mainly due to an increase in the interfacial energy. This result corroborates the common empirical observation that eutectic compositions tend to show good glass-forming ability. We also found that the structural relaxation times are significantly shorter than the characteristic nucleation times, validating the widely used and scarcely tested assumption that relaxation does not play a noticeable role in crystal nucleation. For a stoichiometric glass the fitted diffusion coefficient determining nucleation, D, is lower than that determining the growth of macro crystals, DU, and this difference significantly increases with decreasing temperature. On the other hand, the diffusion coefficient calculated from viscosity, Dη, is close to DU at high temperatures and approaches D in the glass transition range. Finally, after crystallization of the primary phase (lithium disilicate), barium disilicate and more lithium disilicate crystals precipitate in the diffusion zone existing in the residual glass.

Significant strengthening of a lithium disilicate glass by Li+/Na+ exchange at substantially lowered temperature

A lithium disilicate (LD) glass with stoichiometric composition and glass transition temperature (Tg) ∼ 470 °C was ion-exchanged over a wide temperature range below the Tg. The ion-exchange temperatures used were 380 °C or 315 °C in a pure molten NaNO3 bath, and 235 °C in a molten mixed salt bath of 50 mol% NaNO3＋50 mol% KNO3. Temperature-dependences of surface layer morphology and mechanical behavior of the ion-exchanged glass were investigated. It was found that similar surface Na+ concentrations were presented on the glass after the ion-exchange at all the three temperatures for different times. Surface failure occurred on the glass after the treatments at the two higher temperatures for longer time to increase the ion-exchanged depth. The surface failure could be completely avoid by substantially lowering the temperature to 235 °C in the mixed salt bath for much longer time to obtain a similar level of case depth. Similar strengthening effects were exhibited at the three temperatures as the ion-exchanged depth was shallower. With further increasing the depth, however, much more significantly strengthening effect could be achieved at the substantially lowered temperature. Flexural strength of the glass could be enhanced from ∼ 97 MPa to ∼ 520 MPa after the treatment at 235 °C for 128 h. The temperature-dependent effect to strengthen the LD glass by Li+/Na+ exchange was attributed to the combined effect of stress relaxation during ion-exchange and thermal contraction coefficient mismatch between the ion-exchanged surface layer and the internal bulk glass.

The ability of GexGa4S96-x chalcogenide glasses dissolving rare earth probed by x-ray photoelectron spectra analysis

We have synthesized the ternary chalcogenide glasses GexGa4S96-x (x = 22.5, 27, 30, 33, 36), and studied the structural evolution of the glasses by the high-resolution x-ray photoelectron spectra (XPS) technique. We decomposed S 2p, Ge 3d and Ga 3d core XPS spectra into different structural units and found that, with increasing Ge content, part of S-Ge(Ga)-S structure changes into (Ge,Ga)-Ge(Ga)-S, and finally Ge(Ga) clusters appear in the chemically stoichiometric and S-poor glasses. The formation of Ge(Ga) clusters in the chemically stoichiometric and S-poor glasses indicates that, only S-rich Ge–Ga–S glasses can be effectively used as hosts for the doping of rare earth ions, in which no Ga clusters exist and the rare earth ions can be uniformly distributed in the glasses to avoid any possible photoluminescence quenching.