Infrared and MAS NMR studies of potassium borovanadate glasses

Journal of Non-crystalline Solids, 2009

a b s t r a c t Glasses of the system: xBi 2 O 3 -(100Àx)B 2 O 3 (x = 20 to 66 mol%) were prepared and characterized by density, DSC, UV-visible absorption and 11 B MAS-NMR spectroscopy. Glass molar volume increases while the glass transition temperature decreases with Bi 2 O 3 concentration. Densities of some bismuth borate glasses are found to be greater or very close to those of single crystal phases with equal composition. B 11 MAS-NMR studies determined that the fraction of tetrahedrally coordinated borons (N 4 ) is maximum at 42 mol% of Bi 2 O 3 and that there is a local maxima in N 4 at Bi 2 O 3 concentration of 50 mol%. Glasses containing Bi 2 O 3 concentration of 33 mol% and higher show an unusual, intense absorption band just below the optical band gap. Two crystalline phases: Bi 3 B 5 O 12 and Bi 4 B 2 O 9 were prepared by devitrification of glasses and characterized by X-ray diffraction, FTIR and 11 B MAS-NMR studies. Both crystalline phases contained significantly lower N 4 than glasses with equal composition. * The broad absorption band centered at 541 cm À1 is probably the superposition of two absorption peaks at 538 and 550 cm À1 due to Bi 3 B 5 O 12 phase. ** The band at 844 cm À1 could be the superposition of peaks at 839 and 863 cm À1 due to B-O vibrations in Bi 3 B 5 O 12 phase or due to the Bi-O bond vibrations. s-denotes a shoulder.

Materials Chemistry and Physics, 2009

a b s t r a c t Glasses in the system (x/2)Bi 2 O 3 -(x/2)Nb 2 O 5 -(1 − x)Na 2 B 4 O 7 , 0 ≤ x ≤ 0.3, have been prepared by the melt quenching technique. Elastic properties and Debye temperature (Â D ) have been investigated using sound velocity measurements at 4 MHz. The ultrasonic parameters along with the IR spectroscopic studies have been employed to study the role of Bi 2 O 3 , and Nb 2 O 5 on the structure of Na 2 B 4 O 7 glass. The density, the molar volume, the sound velocities, and the glass transition temperatures of these samples have been found to be compositional dependent. The results indicate that Bi 2 O 3 , and Nb 2 O 5 act as a network modifier in the range 0 ≤ x ≤ 0.15, while beyond x = 0.15, Nb 2 O 5 acts as a network former which affects the diborate units that mainly consist the strong borate network. These results are interpreted in terms of the transformation of NbO 6 into NbO 4 , the increase in the number of non-bridging oxygen atoms, and the substitution of longer bond lengths of Bi-O, and Nb-O in place of shorter B-O bond. The observed compositional dependence of the elastic moduli is interpreted in terms of the effect of Bi 2 O 3 , and Nb 2 O 5 on the Boron-coordination number of the glass structure and to the relatively large electron-phonon anharmonic interactions.

OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS

The aim of structural characterization of the V2O5-Bi2O3-B2O3 glassy system is to clarify the role played by the Bi2O3 as an unconventional network former in the structure of the investigated samples. FT-IR absorption spectra present the vibration frequencies characteristic to the both Bi2O3 and B2O3, and, also, the influence of vanadium penthaoxide on the main IR absorption bands. The presence of the main structural units [BiO3], [BiO6], [BO3] and [BO4] and their dependence on the V2O5 content is evidenced. The EPR (Electron Paramagnetic Resonance) spectra contain a single resonance line centered at g ≈ 1.98 with a partially resolved 51 V (I=7/2) hyperfine structure for the samples with high V2O5 content (x > 30 mol %). This fact reveals the existence of the isolated paramagnetic V 4+ ions, whose resonance line overlaps the broad line characteristic to the associated V 4+ ions (for 1≤ x ≤ 20 mol %). 11 B MAS-NMR (Magic Angle Spinning Nuclear Magnetic Resonance) spectra performed on the studied glass samples indicate the changes produced by the addition of vanadium penthaoxide on the coordination of boron atoms. The modification of the relative intensity and position of the resonance lines corresponding to the boron atoms suggests the influence of V2O5 on the local structure of the studied glasses.

Journal of Physics and Chemistry of Solids, 2009

SrO-borovanadate glasses with nominal composition (V 2 O 5 ) 0.5 (SrO) 0.5Ày (B 2 O 3 ) y , 0.0ryr0.4 were prepared by a normal quench technique and investigated by direct current (DC) electrical conductivity, inductively coupled plasma (ICP) spectroscopy, infrared (IR) spectroscopy and X-ray powder diffraction (XRD) studies in an attempt to understand the nature of mechanism governing the DC electrical conductivity and the effect of addition of B 2 O 3 on the structure and electrical properties of these glasses. XRD patterns confirm the amorphous nature of the present glasses and actual compositions of the glasses were determined by ICP spectroscopy. The temperature dependence of DC electrical conductivity of these glasses has been studied in terms of different hopping models. The IR results agree with previous investigations on similar glasses and it has been concluded that similar to SrOvanadate glasses, metavandate chain-like structures of SrV 2 O 6 and individual VO 4 units also occur in SrO-borovanadate glasses. The SrV 2 O 6 and VO n polyhedra predominate in the low B 2 O 3 -containing SrOborovanadate glasses as B substitutes into the V sites of the various VO n polyhedra and only when the concentration of B 2 O 3 exceeds the SrO content do BO n structures appear. This qualitative picture of three distinct structural groupings for Sr-vanadate and Sr-borovanadate glasses is consistent with the proposed glass structure on previous IR and extended X-ray absorption fine structure (EXAFS) studies on these types of glasses. The conductivity results were analyzed with reference to theoretical models existing in the literature and the analysis shows that the conductivity data are consistent with Mott's nearest neighbor hopping model. Analysis of the conductivity data shows that they are consistent with Mott's nearest neighbor hopping model. However, both Mott VRH and Greaves models are suitable to explain the data. Schnakenberg's generalized polaron hopping model is also consistent with temperature dependence of activation energy. However, various model parameters such as density of states, hopping energy, etc. obtained from the best fits were not found to be in accordance with the prediction of the Mott model.

Journal of Physics and Chemistry of Solids, 2007

Potassium-zinc borophosphate glasses were prepared and studied in two compositional series xK 2 O-(50Àx)ZnO-10B 2 O 3 -40P 2 O 5 and xK 2 O-(50Àx)ZnO-20B 2 O 3 -30P 2 O 5 with x ¼ 0, 10, 20, 30, 40 and 50 mol% K 2 O. The replacement of zinc by potassium decreases the density and increases the molar volume of these glasses, whereas glass transition temperature and chemical durability decrease with increasing potassium content. Structural changes were studied by 11 B and 31 P MAS NMR and Raman spectroscopy. The observed changes in the spectra and the properties of the studied glasses can be ascribed to several reasons and namely to the differences in the space occupied by cations Zn 2+ and 2K + , the differences in the electronegativity of zinc and potassium and a large difference in the field strength of Zn 2+ and K + cations and thus higher ionicity of K-O bonds in comparison with Zn-O bonds. r

Journal of Physics D: Applied Physics, 2007

Glasses in the system Na 2−2x B 4−4x Bi x Mo 0.5x O 7−4x , 0 x 0.4, have been prepared by the melt quenching technique. Elastic properties and IR spectroscopic studies have been employed to study the role of Bi 2 O 3 and MoO 3 on the structure of Na 2 B 4 O 7 glass. Elastic properties and Debye temperature have been investigated using sound velocity measurements at 4 MHz. The results showed that the density and the molar volume increase while both sound velocities and the determined glass transition temperatures decrease with increase in x. Infrared spectra of the glasses reveal that the strong borate network consists of diborate units and is affected by the increase in the concentration of Bi 2 O 3 , and MoO 3 . These results are interpreted in terms of the increase in the number of non-bridging oxygen atoms, substitution of longer bond lengths of Bi-O, and Mo-O in place of shorter B-O bond and the change in Na + ion concentration. The results indicate that bismuth and molybdenum ions have been substituted for boron ions as network modifier ions. The elastic moduli are observed to increase with the increase in Bi 2 O 3 and MoO 3 content. This contradiction in the elastic moduli-molar volume relation is attributed to the role of the respective bonds.

Journal of non- …, 2000

Barium borate glasses are reported over an extended glass-forming range from R 0X2 (16.7 mol% BaO) to R 2X0 (66.7 mol% BaO), where R is the molar ratio of barium oxide to boron oxide. The density, T g (glass transition temperature), T x (glass recrystallization temperature), and optical cutos were determined. These data were compared with structural models for the glasses based on nuclear magnetic resonance results of Greenblatt and Bray. The barium borate data were also compared with similar studies of lithium and lead borate glasses. Key results observed include: (1) the density trends are understandable in terms of the abundance of the basic borate groups with the fraction of tetrahedral borons being the most important single factor, and (2) the T g s are anomalously high when compared to either the alkali or lead borate systems. Ó

Journal of Non-Crystalline Solids, 2001

By using rapid cooling we have greatly extended the reported glass-forming ranges in the binary magnesium, calcium, strontium, and barium borate systems. We observed phase separation for low alkaline-earth oxide contents, typically below about 15 mol% alkaline-earth oxide, but we have been able to increase the alkaline-earth oxide limit of glass formation to approximately 60±65 mol%. We have determined the density and the glass transition temperature for a large number of glasses within each of these systems. We compare these data with atomic arrangement studies based on spectroscopy. Using a model derived from the NMR data of Greenblatt and Bray [Phys. Chem. Glasses 8 (5) (1967) 190±193] we have determined the associated volumes of the four basic borate structural units thought to be present in each system. These groups include trigonal borons with three (f 1), two (f 3), and one bridging oxygens (f 4) as well as the tetrahedral boron unit (f 2). We compare these volumes with those obtained from the binary alkali borates. In addition, we have calculated the volumes of the borate groups using Shannon and Prewitt radii. This allowed us to determine and compare the packing fractions of each borate arrangement in the alkaline-earth and alkali borate systems. The T g data display two distinct regions, a high temperature region for the alkaline-earth borates and one for the alkali borates. These regions are separated by approximately 150°C.

The Journal of Chemical Physics, 2016

A series of transition and post-transition metal ion (Mn, Cu, Zn, Pb, Bi) binary borate glasses was studied with special consideration of the cations impact on the borate structure, the cations cross-linking capacity, and more generally, structure-property correlations. Infrared (IR) and Raman spectroscopies were used for the structural characterization. These complementary techniques are sensitive to the short-range order as in the differentiation of tetrahedral and trigonal borate units or regarding the number of non-bridging oxygen ions per unit. Moreover, vibrational spectroscopy is also sensitive to the intermediate-range order and to the presence of superstructural units, such as rings and chains, or the combination of rings. In order to clarify band assignments for the various borate entities, examples are given from pure vitreous B 2 O 3 to meta-, pyro-, ortho-, and even overmodified borate glass compositions. For binary metaborate glasses, the impact of the modifier cation on the borate speciation is shown. High field strength cations such as Zn 2+ enhance the disproportionation of metaborate to polyborate and pyroborate units. Pb 2+ and Bi 3+ induce cluster formation, resulting in PbO n -and BiO n -pseudophases. Both lead and bismuth borate glasses show also a tendency to stabilize very large superstructural units in the form of diborate polyanions. Far-IR spectra reflect on the bonding states of modifier cations in glasses. The frequency of the measured cation-site vibration band was used to obtain the average force constant for the metal-oxygen bonding, F M-O . A linear correlation between glass transition temperature (T g ) and F M-O was shown for the metaborate glass series. The mechanical properties of the glasses also correlate with the force constant F M-O , though for cations of similar force constant the fraction of tetrahedral borate units (N 4 ) strongly affects the thermal and mechanical properties. For paramagnetic Cu-and Mn-borate glasses, N 4 was determined from the IR spectra after deducing the relative absorption coefficient of boron tetrahedral versus boron trigonal units, α = α 4 /α 3 , using NMR literature data of the diamagnetic glasses.

Physical Chemistry Chemical Physics, 2021

Bismuth-containing borate glasses, xBi 2 O 3-(1 À x)B 2 O 3 , were synthesized in the broad composition range 0.20 r x r 0.80 by melting in Pt crucibles and splat-quenching between two metal blocks. Infrared reflectance spectra, measured in the range 30-5000 cm À1 , were transformed into absorption coefficient spectra and then deconvoluted into component bands to probe the glass structure as a function of composition. Integrated intensities of bands above 800 cm À1 were used in combination with mass and charge balance equations to quantify the short-range borate structure in terms of the molar fractions X 4m , X 4o , X 3 , X 2 , X 1 and X 0 for borate units BØ 4 À , BØ 2 O 2 3À , BØ 3 , BØ 2 O À , BØO 2 2À and BO 3 3À , where Ø and O À denote bridging and non-bridging oxygen atoms. Borate tetrahedral units were found to be present in both the meta-borate, BØ 4 À , and ortho-borate, BØ 2 O 2 3À , forms with BØ 4 À constituting the dominating tetrahedral species for 0.20 r x r 0.70. The BØ 2 O 2 3À units prevail at higher Bi 2 O 3 levels (x 4 0.7), and coexist with their isomeric triangular borate species BO 3 3À (BØ 2 O 2 3À " BO 3 3À). The present IR results for the total molar fraction of borate tetrahedral units, X 4 = X 4m + X 4o , are in very good agreement with reported NMR results for the fraction of boron atoms in four-fold coordination, N 4. Besides evaluating X 4m and X 4o , the present work reports also for the first time the fractions of all types of triangular borate species X 3Àn with n = 0, 1, 2 and 3. The IR region below 550 cm À1 was found to be dominated by the Bi-O vibrational activity in coexisting ionic (160-230 cm À1) and distorted BiO 6 sites (330-365 cm À1 and 475-510 cm À1), a result reflecting the dual role of Bi 2 O 3 as glass-modifier and glass-former oxide. The latter role dominates in glasses exceeding 60 mol% Bi 2 O 3 , and is consistent with the extended glass formation in the bismuth-borate system. The structural results were used to calculate the average number of bridging B-Ø bonds per boron center, the average Bi-O and B-O single bond energy, and the atomic packing density of the studied glasses. These properties vary approximately linearly with Bi 2 O 3 content in the three regimes 0.2 r x r 0.4, 0.4 o x r 0.6 and 0.6 o x r 0.83, and contribute collectively to the composition dependence of glass transition temperature.