The structure of lithium,sodium and potassium germanate glasses,studied by Raman scattering

Polarized and depolarized Raman spectra of alkali germanate glasses are given, together with Raman powder spectra of the crystalline compounds Li₂O · 2 GeO₂; 3 Li₂O · 8 GeO₂; 2 Li₂O· 9 GeO₂; Li₂O · 7 GeO₂; 2 Na₂O · 9 GeO₂; K₂O · 2 GeO₂; K₂O · 4 GeO₂ and K₂O · 8 GeO₂.The alkali germanate glasses: xA₂O (1?x) GeO₂ are studied in the composition range 0 < x < 0.333. The vibrational modes observed in the high energy range of the Raman spectra of the crystalline compounds are interpreted in terms of symmetrical and antisymmetrical O-Ge-O and Ge-O^? stretch vibrations. The molecular structure of the germanate glasses is deduced from a comparison of the Raman spectra of the glasses with those of the crystalline compounds, together with a study of the polarization properties of the glass spectra.It is observed that 6-coordinated Ge atoms occur in a network structure which resembles the structures occurring in the crystalline compounds 2 Li₂O · 9 GeO₂ and 2 Na₂O · 9 GeO₂.In the region of 0.18 < x < 0.33 it is found that tetrahedra with one non-bridging oxygen atom are formed. These tetrahedra are probably present in a network as occurs in the crystalline digermanates Li₂O · 2 GeO₂ and K₂O · 2 GeO₂.     

A molecular dynamics study of the role of the cation in modifying the structure of alkali borate glasses

The structures of the full series of alkali borate glasses (M₂O)_x(B₂O₃)_1?x (M = Li, Na, K, Rb and Cs) at two different concentrations, x = 0.14 and x = 0.30, have been investigated by means of molecular dynamics simulations. Additional compositions have also been investigated for the lithium and caesium borate glasses (x = 0.10, 0.20, 0.25, and 0.40). The main experimental trends are well reproduced by the simulations, even if the agreement is not quantitative. Our results indicate that lithium atoms can enter into the matrix of pure vitreous B₂O₃ without inducing large modifications in the B–O network, even at large concentrations. However when the other alkali ions are added to the initial structure, the network opens to accommodate the larger size of the cation. These modifications induce the appearance of a low-Q shoulder or pre-peak, whose intensity increases with increasing alkali concentration as well as with increasing alkali size.     

Mixed alkali glass spectra and structure

The far infrared and Raman spectra of several series of mixed alkali metaphosphate glasses have been investigated as a function of the mole fraction x of the network-modifying ionic oxides in xM₂O(1?x)M₂′O · P₂O₅. The frequencies of the cation-motion bands in the far infrared spectra, which correspond to cationsite vibrations, do not shift with x, indicating that the vibrationally significant local geometry and forces associated with a particular cation are unaffected by the introduction of the second cation into the glass structure. Each Raman-active band due to vibrations of the metaphosphate network occurs at a different frequency for each pure glass (x = 0 or 1), but for mixed alkali glasses only one band occurs for each type of mode and it varies linearly with x. This indicates that the cations in these mixed alkali glasses are homogeneously distributed, there is no significant molecular-level domain formation and the phosphate chains are associated with an averaged cation environment whose effect on the chain modes varies with x. A simple vibrational model is presented which shows that the cation-dependent shifts are due to small changes in network bond angles and variation of the cationsite forces.     

Catalysis and proton-conduction of novel phosphate glasses

The performance of phosphate glasses as a catalyst for water decomposition and a proton conductor was investigated. Glasses with a composition of 30Na₂O–10BaO–30P₂O₅–(30?x)WO₃–xNb₂O₅ (5 < x < 25) decompose water vapor and generate hydrogen at 500 °C. The best decomposition performance was observed on a specimen with the Nb₂O₅ composition of x = 15. A part of hydrogen produced on the glass surface changes to protons by reducing W⁶⁺ ions and penetrates into the glass. The electron is the dominant charge carrier in the electric conduction of W-rich glasses, whereas proton conduction is predominant in Nb-rich glasses in hydrogen atmosphere. A Raman scattering experiment revealed that Nb contributes to depolymerize the –P–O–P– chains in the phosphate glass producing non-bridging oxygen. A possible model was proposed for the water decomposition and proton conduction processes.     

Spectral studies of Sm3+ and Dy3+ doped lithium cesium mixed alkali borate glasses

The effect of host glass composition on the optical absorption and fluorescence spectra of Sm³⁺ and Dy³⁺ has been studied in mixed alkali borate glasses of the type 67B₂O₃ · xLi₂O · (32 − x)Cs₂O (x = 8, 12, 16, 20 and 24). The Judd–Ofelt intensity parameters (Ω₂, Ω₄ and Ω₆) are calculated. The radiative transition probabilities (A), radiative lifetimes (τ_R), branching ratios (β) and integrated absorption cross-sections (Σ) are computed for certain excited states of Sm³⁺ and Dy³⁺ ions for different x values in the glass matrix. Stimulated emission cross-sections (σ_p) are obtained for certain emission transitions of two ions in these mixed alkali borate glasses. These parameters are compared for different x values in the glass matrix. Variation of these parameters with x in the glass matrix has been studied.     

Internal friction and dielectric losses of mixed alkalialkaline-earth metaphosphate glasses

Several series of glasses with the general formula xR₂O · (1 ? x)R′ O · P₂O₅ were prepared and the dielectric loss and internal friction data were taken. The dielectric losses of alkali metaphosphate glasses are greatly reduced by the substitution of alkaline-earth oxide for alkali oxide. Some of the investigated series show a weak minimum in the dielectric loss.The internal friction measurements reveal a high temperature peak, in addition to the single alkali peak and the intermediate temperature peak commonly observed in phosphate glasses. The origin of this high temperature peak is discussed.     

The structure of borate glasses studied by Raman scattering

Raman spectra of alkali and alkaline-earth borate glasses are reported. These spectra are used to discuss the molecular structure of the glasses. The influence of additions of Al₂O₃ on the structure of alkali borate glasses is also presented. The experimental evidence indicates that the same type of groups are present in borate glasses as in crystalline borates. The presence of tetraborate, metaborate, pyroborate and orthoborate groups in borate glasses is strongly suggested by the Raman spectra.     

On the structure of tungstate–tellurite glasses

A continuous network model of xWO₃–(1 − x)TeO₂ glasses is developed, based on quantum-chemical calculation and Raman spectra analysis, in order to relate the structural and vibrational properties with glass composition. The tungstate–tellurite glass network is shown to be formed mainly by structural units of three types, TeO₄ trigonal bipyramids, OTeO₂ trigonal pyramids, and WO₆ octahedra with OW double bonds. Most of the W atoms are found to be incorporated into single OWO₅ octahedra, with no more than several percents of these atoms occurring in 2[OWO₅] paired tungstate centers. The structural and vibrational properties of tungstate–tellurite glasses of several compositions are analyzed by application of the model and a novel interpretation of the Raman spectra is suggested.     

Electrical conductivity studies in single and mixed alkali doped cobalt-borate glasses

The glasses of the type (Li₂O)_x-(CoO)_0.2-(B₂O₃)_0.8−x and (Li₂O)_0.2-(K₂O)_x-(CoO)_0.2-(B₂O₃)_0.6−x were prepared by melt quench technique and their non-crystallinity has been established by XRD studies. The glasses were investigated for room temperature density and dc electrical conductivity in the temperature range 300-550 K. Molar volumes were estimated from density data. Composition dependence of density and molar volume in both the sets of glasses has been discussed. Conductivity data has been analyzed in the light of Mott’s Small Polaron Hopping (SPH) Model and activation energies were determined. Variation of conductivity and activation energy with Li₂O content in single alkali glasses indicated change over conduction mechanism from predominantly electronic to ionic, at 0.4 mole fraction of Li₂O. In mixed alkali glasses, the conductivity has passed through minimum and activation energy has passed through maximum at x = 0.2. This has been attributed to the mixed alkali effect. It is for the first time that a change over of predominant conduction mechanism in lithium-cobalt-borate glasses and mixed alkali effect in lithium-potassium-cobalt-borate glasses has been observed. Various physical and polaron hopping parameters such as polaron hopping distance, polaron radius, polaron binding energy, polaron band width, polaron coupling constant, effective dielectric constant, density of states at Fermi level have been determined and discussed.     

Mixed conductivity in tungstenite–phosphate glasses containing alkali metal ions

The conductivity of glasses in the 50WO₃–(50 ? x)P₂O₅–xA₂O (A = Na, K, Cs) system has been investigated as a function of composition. It is shown that in tungstenite–phosphate glasses containing different alkali metal ions the conductivity decreases with an increase in the alkali metal ion content. A decrease in conductivity is larger for heavier ions and reaches more than seven orders of magnitude in the case of glass containing Cs₂O. This behavior remains in contrast to the literature data on conductivity in transition metal oxide glasses containing alkali metal ions where usually strong conductivity anomalies of several orders of magnitude at certain amount of ions are observed. No necessity of ion–polaron interaction has been pointed out.     

Microstructural analysis of Ga2S3–2MCl (M = K, Rb, Cs) glasses using Raman scattering

Raman scattering spectra of Ga₂S₃–2MCl (M = K, Rb, Cs) glasses have been conducted at room temperature. Based on the analysis of the local co-ordination surroundings of Cs⁺ ions, the similarities and differences of Raman spectra for the glass Ga₂S₃–2CsCl and the bridged molecular GaCl₃ were explained successfully. Through considering the effect of M⁺ ions on mixed anion units [GaS_4?xCl_x] and bridged units [Ga₂S_6?xCl_x] and the corresponding microstructural model, the Raman spectral evolution of the Ga₂S₃–2MCl (M = K, Rb, Cs) glasses was reasonably elucidated.     

Optical absorption of cobalt (II) in binary thallium borate glasses

The optical absorption spectra of cobalt (II) in Tl₂OB₂O₃ glasses have been studied and compared with those in binary alkali borate glasses. In thallium borate glasses cobalt (II) may be present in octahedral and/or in tetrahedral symmetry depending upon the composition of the glass. In low thallium borate glasses cobalt (II) is octahedral while the concentration of tetrahedral cobalt (II) increases with increasing Tl₂O content of the glass; the formation of tetrahedral cobalt (II) becomes noticeable when the concentration of Tl₂O reaches above the critical concentration of about 19 mol %. The ligand field parameters: 10Dq and B have been calculated from the absorption spectra of cobalt (II) in different glasses and it has been found that the Racah parameter, B, is more in Tl₂OB₂O₃ glasses than those in Na₂OB₂O₃ or K₂OB₂O₃ glasses of corresponding molar composition. This indicates that the donor capacity of the BO₄ group in thallium borate glasses is lower than that in alkali borate glasses; this is consistent with the NMR results in Tl₂OB₂O₃ glasses containing less than 20 mol % Tl₂O where three BO₄ groups have been found to form with each Tl₂O unit added.     

Network structure of sodium and potassium borosilicate glass systems

Molar volumes and optical absorption spectra of Ni²⁺ ions were measured for sodium and potassium borosilicate glasses of the compositions xR₂O·B₂O₃·rSiO₂ (0.01 < x < 2.0; r = 1 and 2), where parameters x and r represent respectively the molar ratios R₂O/B₂O₃andSiO₂/B₂O₃. The presence of structural groups was discussed from the results. It was confirmed that addition of an alkali oxide changed BO₃ units to RBO₄ units in the range x < 0.55 for r=1 and x < 0.6 for r=2, while it created ROSiO₃ units in the range $\text{0.55 < x < 0.8}\text{for}\text{r=1}\text{and}\text{0.6}\text{x}\text{?}\text{< 0.9}\text{for}\text{r=2}$. In the range x > 0.8 for r=1 and x > 9 for r=2, ROSiO₃ and ROBO₂ units were created by the alkali addition. Here ROBO₂ units were also formed as a result of self-decomposition of RBO₄ units. The volume of void associated with a BO₄ unit was calculated and compared with the size of sodium and potassium ions. It was shown that sodium ions were small enough to be accommodated in the void whereas potassium ions were too large. This could explain the composition dependence of the molar volume of the sodium and potassium borosilicate glasses in the composition range x < 0.55 for r = 1 and x < 0.6 for r =2.     

FT-IR and Raman study of silver lead borate-based glasses

Glasses from the xMnO · (100−x)[3B₂O₃ · 0.9PbO · 0.1Ag₂O] system with 0 ⩽ x ⩽ 20 mol% have been prepared and studied by means of FT-IR absorption and Raman scattering. We interpreted the spectroscopic data in conjunction with the structural information obtained by X-ray diffraction and scanning electron microscopy (SEM). The X-ray patterns have showed homogenous glasses over the entire compositional range while the SEM pictures have detected metallic silver or Ag₂O clusters dispersed in the glass network. Acting as complementary spectroscopic techniques, both types of measurements, FT-IR and Raman, revealed that the network structure of the studied glasses is mainly based on BO₃ and BO₄ units placed in different structural groups, the BO₃ units being dominant. The influence of manganese-ion content (x), on the N_BO4/N_BO3 ratio evolution was investigated.     

The short-range structure of alkali disilicate glasses by pulsed neutron total scattering

Time-of-flight total neutron scattering experiments using pulsed neutrons with short wavelength produced by an electron linear accelerator have been carried out to measure the structure factors of SiO₂ and M₂O · 2 SiO₂ (M = Li, Na and Li_0.63Na_0.37) glasses over a wide range of the scattering vector up to Q (=4π sin θ/λ) ～480 nm^?1. The Placzek correction has been considered up to the second order in the energy transfer. The Fourier transform of the structure factors yield the pair distribution functions highly resolved in real space, from which the stretching of the SiO and OO atomic distances and the number of bridging and non-bridging oxygen atoms in the alkali disilicate glass have been obtained.     

Halogen Form in Calcium Borate Glasses Containing Iron

Some halogen-doped calcium borate glasses containing iron have been prepared according to the precentage molar composition [(30-X)CaO · xCaX₂ · 10 Fe₂O₃ · 60 B₂O₃)] and annealed. Scanning electron micrographs of heat-treated glasses show that the microstructure consists essentially of two phases. By introducing the halogen ion one of the primary phases (Ca-rich borate) decreases in its volume and a new phase appears and exists as crystalline form. A secondary phase can also be produced which segregates in Ca-rich borate as droplets. Magnetic susceptibility, molar volume, and infrared measurements revealed that at concentrations of CaX₂ < 10 mol% the halide ions may be introduced as modifiers and as network formers at CaX₂ concentrations > 10 mol%.     

Raman spectroscopic study of some lead phosphate glasses with tungsten ions

The structure of xWO₃ · (100 − x)[2P₂O₅ · PbO] glass system with 0 ⩽ x ⩽ 50 mol% was investigated by Raman spectroscopy. The characteristic bands of these glasses due to the stretching and bending vibrations were identified and analyzed by the increasing of WO₃ content. This fact allowed us to identify the specific structural units which appear in these glasses and thus to point out the network modifier role of tungsten oxide for low concentrations and its former role at high concentrations.     

Raman spectroscopy studies of xNa2S+(1−x)B2S3 glasses and polycrystals

The Raman spectra of binary xNa₂S+(1−x)B₂S₃ glasses and polycrystals have been measured for the first time and are used to develop a structural model of the sodium thioborate glasses. The Raman spectra confirm our previous infrared (IR) experimental conclusions that the structure of vitreous (ν-B₂S₃) is comprised of B3(0) groups and six-membered rings. It was also found that as sodium sulfide is added to the glass in the low alkali (x<0.35) glass forming region, the B4 groups are formed at the expense of the B3(0) groups first and then from the six-membered ring groups. The Raman spectra are also consistent with the presence of a pyramidal structural arrangement of B4 groups with trigonally coordinated sulfur atoms. This structure could explain the existence of the super-stoichiometric amounts of B4 groups found using nuclear magnetic resonance (NMR). Glasses in the high alkali region (0.50<x<0.80) progressively change from being comprised of metathioborate rings to being comprised of B3(3) groups. The Raman spectra also confirms the IR spectra which saw no evidence of B3(2) groups in these sodium thioborate glasses.     

Bismuth germanate and bismuth silicate glasses in cryogenic detectors

We present and discuss the results of measurements of conductivity and secondary electron emission coefficient of Bi_xGe_1−xO_2−0.5x (where x=0.13, 0.23, 0.33, 0.47) and Bi_xSi_1−xO_2−0.5x (where x=0.47, 0.57, 0.67) glasses reduced in hydrogen. The surface conductivity of reduced glasses strongly depends on reduction temperature, reduction time and bismuth content. The temperature dependence of the surface conductivity in a high temperature regime is well described by σ∼exp[−(T₀/T)ⁿ] law where n=1/4. The secondary electron emission coefficient of reduced glasses is practically independent of the degree of reduction but increases when binary glasses are modified by alkali ions. The research results confirmed that bismuth silicate and bismuth germanate glasses modified by alkali ions may be good materials for detectors working in cryogenic temperature.     

Study of CaO-WO3-P2O5 glass system by dielectric properties, IR spectra and differential thermal analysis

Dielectric constant ε^′, loss tanδ and ac conductivity σ_ac of 40CaO-xWO₃-(60−x)P₂O₅ (with 0?x?15) glasses are studied over a range of frequencies and temperature. The dielectric breakdown strength of these glasses is also determined at room temperature. The values of dielectric parameters, viz., ε^′, tanδ and σ_ac of CaO-P₂O₅ glasses are found to decrease with the introduction of WO₃ up to 3 mol% and increase with further increase in the concentration of WO₃; the probable reasons for such an increase are identified and explained with the aid of IR spectra and differential thermal analysis of these glasses. The variation of tanδ with temperature at different frequencies of CaO-P₂O₅ glasses has exhibited dielectric relaxation effects with decreasing relaxation intensity with increase in the concentration of WO₃ from 0 to 3 mol%; such relaxation effects seem to have been absent in glasses containing WO₃ beyond 3 mol%. The relaxation phenomenon has been analysed by a pseudo-Cole-Cole plot method and the possible mechanism responsible for such relaxation effects has been suggested.