Spectroscopy,oxidation-reduction behaviour and DC conductivity of a vanadium-containing barium aluminoborate glass

The vanadium(IV)-vanadium(V) equilibrium in a 37.5BaO, 5.0Al₂O₃, 57.5B₂O3 mol% + X mol% V₂O₅ (where X = 0.25?32.5) glass system has been studied as functions of temperature, partial pressure of oxygen and total vanadium concentration of the melt. The vanadium(V)/vanadium(IV) ratio in the melt increased with increasing partial pressure of oxygen, lowering temperature of melting, and with increasing total vanadium content of the melt. With X ? 10, the vanadium(V)/vanadium(IV) ratio became almost independent of the total vanadium content of the melt.With this knowledge of oxidation-reduction behaviour, a series of glasses containing 2.8?32.5 mol% V₂O₅ (at about 4 mol% intervals) and having a constant vanadium(IV)/vanadium(V) ratio (0.17) were prepared. Density, electronic absorption spectrum (both d-d and charge transfer transitions), and ESR of these glasses were measured. Optical and ESR spectra of these glasses indicated the vanadium(IV) to be present as vanadyl ion, VO²⁺; g| decreased monotonically with increasing vanadium content of these glasses, whereas gβ remained unchanged. The charge transfer transition energy due to vanadium(V) decreased, and the extinction coefficient increased by orders of magnitude with increasing vanadium content of the glass; the most striking changes occurred at X ≈ 10 mol%. DC conductivity of these glasses was measured at different temperatures; a plot of log (?/T) versus 1/T produced straight lines. The slope of these lines remained almost constant (39 ± 1 kcal/mol) for the glasses containing up to about 10 mol% V₂O₅; with further increase of V₂O₅ the slope decreased sharply.It has been concluded that the abrupt changes in properties like partial molar volume of V₂O₅, charge transfer spectrum of vanadium(V), activation energy of polaron hopping — all of which occurred around X ≈ 10 mol% — is due to a major change in the nature of vanadate groups rather than vanadium(IV) in these glasses.     

Raman spectra and structure of sodium aluminogermanate glasses

Polarized Raman spectra of x NaAlO₂·(100 ? x) GeO₂ glasses (x = 0, 5, 10, 15, 20, 25, 33, 42, and 50) are presented. Analyses of the Raman data indicate that the aluminogermanate glasses have three-dimensional network structures consisting of interconnected AlO₄ and GeO₄ tetrahedra; Na⁺ ions are present in cavities and charge balance the Al³⁺ ions. Systematic changes are observed in the frequencies, intensities and polarization characteristics of spectral bands with variations in the NaAlO₂ content of these glasses. The antisymmetric stretching mode [ν_as (TOT), where T = Al, Ge] in the high-frequency region of the spectra (800–1000 cm^?1) appears as a doublet consisting of well-defined bands in the spectra of glasses along the entire join. Both components of the high-frequency doublet shift to a lower frequency with increasing NaAlO₂ content, indicating that the ν_as (GeO₄) and ν_as(AlO₄) stretching modes are coupled. The variations in the TO force constants and TOT bond angles with change in composition most likely cause the bands to shift. The frequencies of the Raman bands of sodium aluminogermanate glasses are compared with those of the corresponding bands in isostructural sodium gallogermanate glasses. On the basis of this comparison, the origin and delocalization of the vibrational modes producing characteristic Raman bands in the spectra of these glasses are discussed. The changes observed in the Raman spectra of aluminogermanate glasses with variation in NaAlO₂ content are analogous to those observed in the spectra of glasses along the NaAlO₂SiO₂ join.     

EPR study of crystalline and vitreous forms of the Li2OAl2O3SiO2 system

The epr spectra of V⁴⁺ and radiation centres have been studied in β-eucryptite (LiAlSiO₄), β-, γ-spodumene (LiAlSi₂O₆) and in glasses prepared by the fusion of the single crystals. It is shown that the electronic structures of the vitreous state in the Li₂OAl₂O₃SiO₂ system and that of the crystalline forms differ considerably. The change of the electronic structure on crystallization is not direct, but is realized through the intermediate state whose electronic structure differs from that of glasses and crystals.     

Vanadium contribution in different glasses in view of the ligand field theory

Spectrophotometric and magnetic studies were carried out on different glasses containing known amounts of V₂O₅ in order to throw some light on the valence states and coordination of vanadium in such glasses (in view of the ligand field theory).The results obtained were found to be in agreement with those of aqueous solutions, e.g. [V(H₂O)₆]³⁺, [VO(H₂O)₅]²⁺, and VO²⁺ which showed a slight difference in band positions due to the distortion as well as the difference between the ligand field strength of glass and water. Generally it may be concluded that vanadium is present in borate and silicate glasses in three possible valencies, V⁵⁺, V⁴⁺ and V³⁺, while in phosphate glasses it is generally present in two valencies V⁴⁺ and V³⁺.     

Bond character and properties of the mixed alkali system Na2OK2OAl2O3SiO2

The mixed alkali glass system Na₂OK₂OAl₂O₃SiO₂ was investigated. Density, transformation temperature, refractive index, and chemical durability were studied. Optical absorption and ESR spectra of the CuO-doped glasses were determined.Calculations of the polarizability of O^2?, bonding parameters of the Cu²⁺ complex, and the packing density are presented. It was found that for the mixed alkali glasses, the oxygen- alkali bond has a more ionic character than expected from additivity. This fact enables the non-linear changes of the refractive index, of the shift of the Cu²⁺ absorption band, and of the covalency to be interpreted as the Na mole fraction is varied. It is also possible to explain qualitatively the density, T_g and chemical durability non-linear variations with change of the Na content by the ionicity deviations of the bond character and the postulated pairs of Na⁺ and K⁺ ions in the mixed alkali glasses.     

Infrared spectra and structure of superionic conducting glasses in the system AgIAg2OMoO3

Thin blown films of glasses with the mole ratio Ag₂O/MoO₃ = 1 in the system AgIAg₂OMoO₃ (or the pseudobinary system AgIAg₂MoO₄) show three absorption bands in the range 4000-200 cm^?1; 875 cm^?1 (w), 780 cm^?1 (s), and 320 cm^?1 (m, b), which are characteristic of tetrahedral MoO₄^2? ions. The glasses with the ratio Ag₂O/MoO₃ < 1 have two additional bands at 600 cm^?1 (w) and 450 cm^?1 (vw), which are characteristic of condensed ions of MoO₄ tetrahedra, probably Mo₂ O₇^2? ions. These glasses are thus composed of Ag⁺, I^?, MoO₄^2?, and probably Mo₂O₇^2? ions, and classified as “ionic” glasses containing one type of cations. The presence of partial covalency in the Ag⁺… ^?OMo link and the influence of ion exchange of Ag⁺ with K⁺ on IR spectra are discussed. The molar volume of the glasses with the ratio Ag₂O/MoO₃ = 1 is primarily determined by a fairly dense packing of the constituent anions, I^? and MoO₄^2?.     

NMR studies on rapidly solidified SiO2Al2O3 and SiO2Al2O3Na2O-glasses

²⁷Al and ²⁹Si MAS NMR studies were performed on roller-quenched SiO₂Al₂O₃-glasses with Al₂O₃ contents ranging from 10 to 60 mol% and on SiO₂Al₂O₃Na₂O glasses containing 10 mol% Al₂O₃ and 2.5 to 10 mol% Na₂O. Pure aluminium silicate glasses show NMR peaks at 0, 30 and 60 ppm. The frequency distribution of the different Al-sites is not affected by the glass composition. In glasses of the system SiO₂Al₂O₃Na₂O the 30 ppm peak decreases to zero as the Na₂O content increases. The 30 ppm peak is assigned to distorted triclustered AlO-tetrahedra, rather than to fivefold coordinated Al. Triclustering of tetrahedra may provide for charge neutrality in glasses with molar excess of Al₂O₃ over Na₂O. As charge balance is increasingly achieved by addition of alkali ions, the tendency of tetrahedral triclustering is reduced, reflected by the disappearance of the 30 ppm peak in glasses containing ≥ 7.5 mol% Na₂O.     

Raman spectroscopy of calcium aluminate glasses and crystals

Solar furnace melting and fast-quench techniques have been used to prepare calcium aluminate glasses from 75 mol% CaO to 82 mol% Al₂O₃, which have been studied by Raman spectroscopy. The CaAl₂O₄ glass spectrum may be interpreted in terms of a fully-polymerized network of tetrahedral aluminate units, which is depolymerized on addition of CaO component analogous to binary silicate systems. The spectra of glasses with higher alumina content than CaAl₂O₄ may not be simply interpreted and a structural model is proposed which would be consistent with the glass spectra and with observed crystal structures along the CaAl₂O₄Al₂O₃ join. This model suggests formation of highly condensed aluminate tetrahedral on initial addition of alumina, with the appearance of aluminate polyhedra of higher average coordination at higher alumina content. Similar high coordination polyhedral are also suggested for a limited composition range along the CaOCaAl₂O₄ join. These interpretations are compared with the results of a previous study in the SiO₂Al₂O₃ glass system.     

Radiation effects in ZrF4 based glasses: I. Electron spin resonance

Binary ZrF₄BaF₂, HfF₄BaF₂, and multicomponent ZrF₄ based glasses have been studied by electron spin resonance spectroscopy after liquid-nitrogen-temperature X-irradiations and room-temperature γ-ray irradiations. Polycrystalline BaF₂, ZrF₄ and HfF₄ were investigated in the same manner for comparison purposes. Isochronal anneal experiments have distinguished several resonances in the glasses which are characterized as due to F₂^? molecular ions, F⁰ interstitial atoms and Zr³⁺ and Hf³⁺ ions. Two more signals are tentatively assigned as due to a hole trapped on several fluorine ions and a hole trapped on an oxygen impurity. In lead-containing glasses the formation of Zr³⁺ is strongly inhibited, presumably due to the competitive formation of Pb⁺ ions. However, the radiation production of Pb³⁺ ions was independently observed, thus suggesting that the Pb²⁺ ions may serve as traps for charge carriers of either sign. No defects were detected which could be explicitly associated with other cationic constituents, such as La³⁺, Al³⁺, or Li⁺.     

Spectral investigations on VO2+ ion doped in CaO–SrO–Na2O–B2O3 glass systems

Glasses of the (20 ? x)CaO–xSrO–(20 ? y)Na₂O–60B₂O₃ ? y (CSNB) system with (5 ≤ x ≤ 15) mol% and y = 0.1 mol% of V₂O₅ were characterized by X-ray diffraction (XRD), EPR (Electron Paramagnetic Resonance), Optical absorption Spectra and FT-IR (Fourier transform Infrared Spectroscopy) studies. EPR spectra of all the glass samples exhibit resonance signals characterstic of VO²⁺ ions. The values of spin-Hamiltonian parameters indicate that the VO²⁺ ions in CSNB glasses were present in octahedral sites with tetragonal compression and belong to C_4v symmetry. Spin-Hamiltonian parameters ‘g’ and ‘A’ were evaluated. The Optical band energy (E_opt) and Urbach energy (ΔE) were calculated from their ultra violet edges. By correlating EPR and Optical data the molecular orbital coefficients have been evaluated. IR spectra of these glasses were analyzed in order to identify the contribution of each component to the local structure that determines the physical properties of these glasses.     

The study of interactions between iron and vanadium ions in semiconducting barium-vanadate glasses doped with Fe2O3

Semiconducting barium-vanadate glasses doped with Fe₂O₃ ranging from 0.1 to 10 wt% were studied. We made attempts to understand features of an incorporation of the impurity ions into the host matrix. EPR, magnetic susceptibility, dc-conductivity and the Mössbauer effect were investigated.It was established that iron entered into the host as Fe³⁺·Fe³⁺ and V⁴⁺ ions formed associates coupled by dipole-dipole interactions for low Fe₂O₃ contents in the glass. The V⁴⁺?Fe³⁺ and Fe³⁺?Fe³⁺ pairs co-existed for all glasses. The contribution of Fe³⁺?Fe³⁺ interactions increased with increasing Fe₂O₃ content. The deviation from paramagnetic behaviour was observed for glasses with 8–9 wt% Fe₂O₃. It was attributed to presence of fine crystalline magnetic particles.The iron impurity induces no considerable changes in the dc-conductivity of the glass. The concentration dependence of dc-conductivity exhibits a minimum of about 5–6 wt% Fe₂O₃.     

Raman spectroscopic study of scandium in sodium silicate glasses

Glasses in the Na₂OSiO₂Sc₂O₃ system have been studied by Raman and difference Raman spectroscopy. Addition of Sc₂O₃ to sodium silicate glasses results in new vibrational bands at 1025 cm^?1 and 360 cm^?1. The high frequency band is interpreted to be due to Sc⁺³ quasi-complexes formed by Sc⁺³ ions coordinated by SiO₄^?4 tetrahedra having non-bridging oxygens. The discrete character of the scandium-produced bands implies incipient separation of Sc⁺³-enriched silicate structures from purely silicate structures.     

Optical spectra of sodium phosphate glasses

The visible, infrared, and ultraviolet spectra of seven glasses in the Na₂OP₂O₅ binary system were obtained. Changes in spectra were related to structural changes occurring the glass. Dopant ions Co²⁺, Cu²⁺, and Pb²⁺ were used to infer structural changes occurring as the metaphosphate composition was crossed going from pyrophosphate compositions into ultraphosphate compositions. The presence of chemically bound water was found to cause the overall modifier-to-former ratio to remain constant for glasses containing greater than 65 mol.% P₂O₅.     

The investigation of the magnetic interaction between Cu2+ and V4+ ions in x(CuO·2V2O5)(1 − x)[2B2O3·K2O] glasses

EPR and magnetic susceptibility measurements have been performed on (CuO·2V₂O₅)(1?x)[2B₂O₃·K_2O] glasses with 0 ? x ? 40 mol. %.For x < 10 mol.%, both Cu²⁺ and V⁴⁺ ions are present mostly as the isolated species. The values of MO coefficients indicate a high covalent degree of the transition metal (TM)-oxygen bonds. Also, the EPR parameters suggest the presence of strong (TM)-oxygen bonds along the 0_z axis, which lead to an octahedral (Oh) symmetry component at TM ions sites.In the case of 10 ? x ? 40 mol.%, the dipole-dipole and superexchange interactions occur between transition metal ions, which determine a broad resonance line at $\text{g ? 2}$. The strong interactions between Cu²⁺ and V⁴⁺ ions give rise to the exchange coupled Cu²⁺ V⁴⁺ pairs in the studied glasses with x > 10 mol.% (y > 3.9 mol.%).     

Optical properties and valence change of samarium ions in a sol–gel Al2O3–B2O3–SiO2 glass by femtosecond laser irradiation

10Al₂O₃–5B₂O₃–85SiO₂–xSm₂O₃ glasses were prepared by the sol–gel method. The emission spectra of the glasses indicate that the quench concentration of the Sm³⁺ ions is about 0.2 mol%. The emission spectra of the glasses after high-temperature treatment with H₂ gas exhibit the coexistence of the Sm³⁺ and Sm²⁺ ions. We observed the strong emission line of the Sm²⁺ ions and the emission band of the non-bridging oxygen hole center when the glasses were exposed to a femtosecond laser. It indicates that some Sm³⁺ ions were reduced to Sm²⁺ ions by femtosecond laser pulses and non-bridging oxygen hole centers were formed. The ⁵D₀–⁷F₀ emission line of the Sm²⁺ ions by femtosecond laser irradiation shows a red shift, compared with the emission of the Sm²⁺ ions by reduction with H₂ gas. The strong absorption band and weak, sharp absorption lines in the range from the UV to IR come from charge transfer and the transition from the ⁶H_5/2 state to the various excited states of the Sm³⁺ ions. The reduction mechanism of Sm³⁺ ions is discussed.     

Nuclear magnetic resonance studies of the glasses in the system Na2OB2O3SiO2

The ¹¹B NMR spectra have been used to study the structure of glasses in the system Na₂OB₂O₃SiO₂. The fraction of BO₄ units, and the fraction of BO₃ units with one or two nonbridging oxygens, are measured and analyzed according to a structural model. The results indicate that: (1) for a sodium oxide to boron oxide ratio of 0.5 or less, the Na⁺¹ ions are attracted primarily by the borate network; therefore, the ternary glasses can be viewed as binary sodium borate glasses diluted by SiO₂; (2) when the sodium oxide to boron oxide ratio exceeds 0.5, the additional Na₂O results in the formation of [BSi₄O₁₀]^?1 units at the expense of diborate and SiO₄ units. In this process, Na⁺¹ ions are still taken up only by the borate network. After all the available SiO₄ units are consumed to form [BSi₄O₁₀]^?1 units, additional Na⁺¹ ions are proportionally shared between the borate and silicate networks.     

High temperature ESR study of Fe(III) in various vitreous matrices

The Raman spectra of binary high-silica glasses have been studied. The main peaks at 808 cm^?1 and 710 cm^?1 in vitreous B₂O₃ and vitreous P₂O₅, respectively, are greatly reduced in binary high-silica glass, whereas a peak at 425 cm^?1 due to GeOGe vibration and a peak at 1320 cm^?1 due to P = O vibration remain strong, increasing in intensity with decreasing SiO₂ concentration. In the stimulated Raman spectra of a P₂O₅-SiO₂ glass fiber pumped by a mode-locked and Q-switched Nd:YAG laser at 1.064 μm, strong Stokes emissions due to the P = O vibration have been observed at 1.24 μm and 1.48 μm. In the spectra for a GeO₂-SiO₂ glass fiber, four narrow-width Stokes emissions due to the GeOGe vibration have been observed at 1.115, 1.172, 1.235 and 1.305 μm.     

Raman study of the structure of glasses along the join SiO2GeO2

In order to better understand the distribution of tetrahedra in multicomponent tetrahedral network structures of melts and glasses, we have investigated the Raman spectra of binary SiO₂GeO₂ glasses. We compare the Raman spectral features of the end-member glasses and discuss their vibrational origins. The mixing of GeO₂ and SiO₂ melts results in a continuous random network structure of TO₄ tetrahedra (T  Si, Ge) in the glass. Raman bands corresponding to the asymmetric stretch (v_as) of oxygen in GeOGe, SiOSi and SiOGe bonds are observed in the glasses having intermediate compositions along the SiO₂GeO₂ join. The presence of three distinct v_as (TOT) bands in the spectrum of a glass having Si/Ge one reveals that a considerable degree of SiGe disorder exists in the glass. The presence of a single symmetric oxygen stretching band in the spectra of binary SiO₂GeO₂ glasses indicates that the symmetric stretch modes (v_s) of oxygen in SiOSi, SiOGe and GeOGe bonds are strongly coupled. An observed decrease in the halfwidth of the v_s (TOT) band in the spectra of SiO₂GeO₂ glasses with increasing concentration of GeO₂ may be attributed to a decrease in the average TOT bond angle and a predominance of six-membered ring structures. Results of the present study support the assignment of the bands in the 900–1200 cm^?1 region of the alumino-silicate glasses, spectra to the v_as(AlOSi) and v_as(SiOSi) modes. In contrast to the alumino-silicate glasses, however, the SiO₂GeO₂ glasses have a much higher degree of disorder of the network-forming cations.     

Raman spectroscopic investigation of the structure of silicate glasses (II). Soda-alkaline earth-alumina ternary and quaternary glasses

Raman spectra of some ternary and quaternary glasses in the system Na₂OCaOMgOAl₂O₃SiO₂ are presented. The spectra are interpreted in terms of the structural alteration of the glass as the composition is altered from the binary end members to more complicated glasses. Addition of CaO and MgO to soda-silica glasses act only to increase the disorder of the network slightly. Addition of Al₂O₃ greatly modifies the network. In some soda-lime-aluminosiliscate compositions an estimate can be made of the amount of aluminum in four- and six-fold coordination. It is shown that the amounts of four- and sixfold coordinated aluminum depend on the glass composition.     

The role of vanadium valence states and coordination on electrical conduction in lithium iodide borate glasses mixed with small concentration of silver iodide

LiI–AgI–B₂O₃ glasses mixed with different concentrations of V₂O₅ (ranging from 0 to 1.0 mol%) were prepared. Electrical and dielectric properties over wide ranges of frequency (10^?2–10⁷ Hz) and temperature (173–523 K) have been studied. Additionally spectroscopic properties viz., optical absorption and ESR spectra have been investigated. The optical absorption and ESR studies have revealed that vanadium ions do exist in both V⁴⁺ and V⁵⁺ states and the redox ratio is the highest in the glasses containing 0.8 mol% of V₂O₅. The results of conductivity measurements have indicated that there is a mixed conduction (both ionic and electronic). The ionic conduction seems to be dominant over polaron hopping only in the glasses containing V₂O₅ more than 0.8 mol% of V₂O₅. The impedance spectra have also indicated that the conduction is predominantly polaronic in nature. The frequency and temperature dependence of the electrical moduli as well as dielectric loss parameters have exhibited relaxation character attributed to the vanadyl complexes. The relaxation effects have been analyzed by the graphical method and from this analysis it has been established that there is a spreading of relaxation times. The results have been further discussed quantitatively in the light of different valance states of vanadium ions with the aid of the data on spectroscopic properties.