A XANES study of the valence state of vanadium in lithium vanadate phosphate glasses

Valence states of vanadium in Li₂O-V₂O₅-P₂O₅ electronic-ionic conducting glasses have been studied by X-ray absorption spectroscopy. The composition dependence of the XANES range of the spectra has been used to estimate relative abundance of the vanadium atoms in V⁴⁺ or V⁵⁺ charge states. This information is important for analysis of electronic transport in these glasses, which occurs via electronic hopping between aliovalent vanadium centers. It was found that in samples with the lowest V₂O₅ contents (10 mole% of V₂O₅) a vast majority of vanadium ions are in V⁴⁺ charge state. At higher V₂O₅ contents the proportion of V⁴⁺ ions decreases at the expense of V⁵⁺ ones, but remains substantial even in glasses with the highest contents of V₂O₅.     

Thermodynamics of oxidation-reduction of vanadium in borate glasses

The V⁴⁺ V⁵⁺ equilibrium in a 2BaO-3B₂O₃ glass has been studied as functions of temperature, at a fixed partial pressure of oxygen, and total vanadium concentration of the melt. The optical and esr spectra of some of these glasses have also been studied to characterize the V⁴⁺ and V⁵⁺ centres. From the linear temperature dependence of the redox concentration ratio, the activation energy for the redox reaction has been determined at each concentration. The plot of logarithm of the ratio of the activity coefficients and also of the enthalpy of the redox reaction against composition showed considerable changes around 22 mol% V₂O₅. Structural changes associated with both V⁴⁺ and V⁵⁺ ions are suggested to account for such behaviour, which was also suggested from a previous study of the conductivity behaviour of such glasses. The optical and esr spectra showed considerable changes in the V⁴⁺-O and V⁵⁺-O bonding, but no sudden change was observed around 22 mol% V₂O₅.     

Seebeck coefficient of V2O5-R2O3-TeO2 (R = Sb or Bi) glasses

The thermoelectric power of glasses in the systems V₂O₅-Sb₂O₃-TeO₂ and V₂O₅-Bi₂O₃-TeO₂ was measured at temperatures in the range 373–473 K. The glasses in both systems were found to be n-type semiconductors. The Seebeck coefficient, Q, at 473 K was determined as –192 to –151 VK^–1 for V₂O₅-Sb₂O₃-TeO₂ glasses, and –391 to –202 VK^–1 for V₂O₅-Bi₂O₃-TeO₂ glasses. For these glasses in both systems, Heikes' formula was satisfied adequately for the relationship between Q and In [C _v/(1-C_v)] (C _v = V⁴⁺/V_total, C _v is the ratio of the concentration of reduced vanadium ions), and discussions confirmed small polaron hopping conduction of the glasses in both systems. Mackenzie's formula relating to Q and V⁵⁺/V⁴⁺ was also applicable to the glasses in both systems, and it was concluded that the dominant factor determining Q was C _v.     

Spectroscopic,electrical and EPR studies of binary semiconducting oxide glasses containing 50 mol% V2O5

Recent measurements on the V₂O₅-GeO₂ glass system consisting of an equimolar mixture of V₂O₅ and GeO₂ revealed that increase in electrical conductivity of these glasses upon annealing could be attributed to the increase in V⁴⁺ and V³⁺ content which accompanied the microstructure formation. In the present work we report a similar study on V₂O₅-TeO₂ and V₂O₅-P₂O₅ glass systems. It was found that in tellurite glass V³⁺ content increased upon annealing and V⁴⁺ content remained unchanged. In phosphate glass some increase in V⁴⁺ and no significant change in V³⁺ contents were observed. V³⁺ and V⁴⁺ contents in glasses could be best estimated from optical and electron paramagnetic resonance spectra, respectively.     

The control of the electrical conductivity of germanium vanadate glasses by the admixture of chlorine during preparation

The electrical conductivity of germanium vanadate glasses depends on the relative concentrations of V⁴⁺ and V⁵⁺ ions. It is found that by adding VCl₃ to the melt when the glass is formed, the added chlorine which acts as an oxidizing agent alters the ratio of concentration of vanadium ions and thus the conductivity. The optical absorption coefficients and d.c. conductivities of germanium vanadate glasses are measured as functions of VCl₃ content. It is found that the activation energy for conductivity increases with chlorine content, the increase of the activation energy corresponding to the change in optical gap energy. It is considered therefore, that the principle of the addition of a strong oxidizing agent to the glass to alter the reduced valency ion ratio may have general application in the control of electrical conductivity in transition metal ion glasses.     

Optical and ESR investigation of borate glasses containing single and mixed transition metal oxides

The optical and electron spin resonance (ESR) spectra of barium borate glasses, containing the oxides of V, Fe and Cu separately and in mixed proportions, have been studied. The optical spectra of the single transition metal (TM) oxide glasses showed the usual features, while those for the mixed glasses showed single bands without showing individual features of the single TM oxide glasses. However, the linear plots of optical density against composition revealed the presence of two valence states for each TM element, and this was confirmed by ESR results as well. The ESR spectra of the mixed glasses showed a complicated interaction pattern for two different TM ions, in comparison with those of the glasses containing a single TM ion. For the Fe-V glasses, the progressively vanishing hyperfine structure of the VO²⁺ complex with increasing addition of iron oxide is discussed in terms of nuclear spin relaxation, cross-relaxation between two spin systems and spin diffusion within the vanadium spin system. The covalency of the VO²⁺ complex and the number of distorted Fe³⁺ ions were found to decrease with increasing addition of Fe₂O₃ replacing V₂O₅. Similar features were noted for the Cu-V glasses; the spectra of Cu-Fe glass also showed a strong interaction between two different TM ions. It has been suggested that all the possible four valence states (for a given mixed glass) from two different TM elements are present, and that pairing of two different TM ions from two dissimilar TM elements occurs, facilitating the formation of associates (e.g. V⁴⁺-O-Fe³⁺).     

Optical absorption spectra of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

The optical absorption spectra of evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ thin films have been studied. For higher photon energies, the absorption is found to be due to a direct forbidden electronic transition process from the oxygen 2p band to the vanadium 3d band in a similar way to that observed in crystalline V₂O₅. The exponential behaviour of absorption edge for lower photon energies is attributed to the electronic transitions between the tailed-off d-d states corresponding to V⁴⁺ ions. For co-evaporated V₂O₅/B₂O₃ films the optical energy gap is observed to increase with the increase in V₂O₅ content of the composite films.     

EPR and TPR investigation of the redox properties of vanadia based ceria catalysts

Vanadium cerium oxides, with different V/Ce atomic ratios, were prepared using the impregnation method and calcined under air at 500°C. Physicochemical studies have shown that at low vanadium content, polymeric V-O-V chains are stabilized on the ceria surface. Increasing the vanadium content tends to favor the formation of the CeVO₄ and V₂O₅ phases. The redox properties of these oxides have been simultaneously investigated by TPR/TPO and EPR techniques. V-O-V chains and V₂O₅ species are more easily reducible than the CeVO₄ phase. The reduction of V₂O₅ to V₂O₃ proceeds in several steps, the intermediate species being V₆O₁₃, VO₂ and V₅O₉. The reduction of V₂O₅ species interacting with ceria support leads to VO oxide. EPR measurements performed at T = −269°C have permitted to observe progressively different signals of V⁴⁺ in addition to vanadium ions in V²⁺ (3d³) paramagnetic configuration. This attribution is based on an EPR signal at g = 3.956 with eight well resolved hyper fine lines (A = 96 Gauss), which may be attributed to the perpendicular components of one of the fine transitions corresponding to the V²⁺ spectrum. At high reduction temperature, CeVO₄ phase leads in one step to CeVO₃ and a continuous and partial reduction of CeO₂ into Ce₂O₃ is observed. Re-oxidation process shows that polymeric V-O-V chains, easily reducible, are hardly re-oxidized whereas V₂O₅ species, present in the high vanadium loading samples, are easily re-oxidized at low temperatures. However, redox processes seem to be reversible.     

EPR study of vanadium-loaded yttrium-cerium-zirconium solid solutions

5YO_1.5-10CeO₂-85ZrO₂ solid solution doped with vanadium was studied by means of electron paramagnetic resonance (EPR). The nature of the vanadium species formed by treatment of samples with O₂, CO, and H₂ was investigated. Three types of paramagnetic V⁴⁺ ions found in samples evacuated at 723 K have been attributed to tetragonally compressed octahedral complexes of vanadyl type with different interionic V–O distances. Two of them are supposed to be located on the surface of the oxide support, whereas the third type is in the bulk of the solid. EPR spectroscopy of all CO reduced samples showed the presence of isolated mononuclear V⁴⁺ species. Reducing treatment of the samples with H₂ also provoked the formation of aggregated V⁴⁺ species. The later species formed either by the agglomeration of surface isolated V⁴⁺ ions during high-temperature treatment or the reduction of polymerized V⁵⁺ species present in as-prepared samples. Obtained results can be used for the better understanding of vanadium effect on catalysts properties.     

Non-linear magnetic susceptibility of some vanadate glasses

The first measurements of the anomalous variation of d.c. magnetic susceptibility of the (50 – x) P₂O₅-xM-50V₂O₅ (M = Bi₂O₃ and Sb₂O₃, and x=0 to 40 mol% M) oxide glasses around 20 to 30 mol% M are reported here. Similar anomalous behaviour was also observed in the electrical and other physical properties of the glasses (reported in the previous paper). Like electrical and dielectric properties, this anomaly in the magnetic properties was also found to be mostly associated with the anomalous variation of the V⁴⁺/V⁵⁺ ratio (around 20 to 30 mol% M) with the concentration of M. From the temperature (80 to 300 K)-dependent magnetic susceptibility data the V⁴⁺ ion concentrations have also been calculated, which agree quite well with those obtained from chemical analysis, and the small discrepancy is attributed to the presence of V³⁺ and/or V²⁺ ions in the glass.     

Magnetic susceptibility of amorphous and heat-treated vanadate glasses

X-ray diffraction and magnetic susceptibility measurements were used to study the effect of addition of iron on the crystallization process and magnetic properties of the vanadium borophosphate system before and after heat treatment. The glass composition was 78 mol% V₂O₅-15 mol% P₂O₅-7 mol% B₂O₃, the Fe₂O₃ was added with concentration ranging from 0.05 up to 1 mol%. The X-ray diffraction showed that the V₂O₅ was the only phase separated during the heat treatment process. The intensity of the characteristic V₂O₅ peaks increased with increasing Fe₂O₃ content. The magnetic susceptibility was found to decrease for the sample containing 0.05 mol% Fe₂O₃. This could be explained by the presence of most of the iron ions in the ferrous state and the presence of a covalent bond between the ferrous ions and the oxygen ions. An increase in the magnetic susceptibility was found in samples containing an iron content of more than 0.05. This increase could be explained according to the change of Fe²⁺ ions to Fe³⁺ which has higher paramagnetic behaviour.     

Structural investigations of V<Subscript>2</Subscript>O<Subscript>5</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript>–CaO glass system by FT-IR and EPR spectroscopies

xV₂O₅·(100 − x)[0.7P₂O₅·0.3CaO] glass system was obtained for 0 ≤ x ≤ 35 mol% V₂O₅. In order to obtain information regarding their structure, several techniques such as X-Ray diffraction, FT-IR, and EPR spectroscopies were used. X-Ray diffraction patterns of investigated samples are characteristic of vitreous solids. FT-IR spectra of 0.7P₂O₅·0.3CaO glass matrix and its deconvolution show the presence in the glass structure of all structural units characteristic to P₂O₅. Their number are increasing for x ≤ 3 mol% V₂O₅ then, for higher content of vanadium ions, the number of phosphate structural units are decreasing leading to a depolymerization of the structure. The structural units characteristic to V₂O₅ were not evidenced but their contribution to the glass structure can be clearly observed. EPR revealed a well resolved hyperfine structure (hfs) typical for vanadyl ions in a C_4v symmetry for x ≤ 3 mol% V₂O₅. For 5 < x < 20 mol% V₂O₅ the spectra show a superposition of two EPR signals one due to a hfs structure and another consisting of a broad line typical for associated V⁴⁺–V⁴⁺ ions. For x ≥ 20 mol% V₂O₅ only the broad line can be observed. The composition dependence of the line-width suggests the presence of dipole–dipole interaction between vanadium ions up to x ≤ 5 mol% V₂O₅ and superexchange interactions between vanadium ions for x > 5 mol% V₂O₅.     

Oxidation-reduction equilibria of vanadium in CaO-SiO2, CaO-Al2O3-SiO2 and CaO-MgO-SiO2 melts

A series of redox studies of vanadium have been carried out in CaO-SiO₂, CaO-MgO-SiO₂ and CaO-Al₂O₃-SiO₂ melts/slags equilibrated over oxygen partial pressures (pO₂) range 10^–2–10^–9 atm at 1600°C. V₂O₅ level was varied from 1–5 mol%. Three different melt basicities and alumina contents were investigated. Magnesia content was varied between 3.5 and 4.9 wt%. A newly developed analytical technique based upon electron paramagnetic resonance (EPR) spectroscopy was successfully applied to these melts. Two redox equilibria corresponding to V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ pairs followed the O-type redox reaction over the oxygen partial pressure range investigated. Higher oxidation states of vanadium were stabilized with increasing basicity of slags. Two redox pairs coexisted within oxygen pressure region 10^–4–10^–6 atm. However, redox ratios did not indicate clear trends with increasing V₂O₅ content in CaO-SiO₂-V₂O₅ system. In CaO-SiO₂-Al₂O₃-V₂O₅ slags, a slight increase in redox ratios (V³⁺/V⁴⁺) was obvious when alumina quantity was changed from 3.22 to 5.44% at a basicity ratio 1.3, indicating an increase in slag acidity. CaO-MgO-SiO₂-V₂O₅ slags showed a sharp decrease in redox ratios (V⁴⁺/V⁵⁺) between 10^–2–10^–6 atm with addition of 3.5 wt% MgO, due to increasing free oxygen ions in slags.     

Effect of annealing on the electrical and optical properties of V2O5-GeO2 glasses

Increases in the electrical conductivities of vanadium germanate glasses on annealing have been reported recently in the literature. The increases were attributed to the formation of microstructure on annealing. In the present work we report a study of the V₂O₅-GeO₂ glass system using electron paramagnetic resonance, optical absorption, differential scanning calorimetry and electron diffraction techniques. The V₂O₅-GeO₂ glass system consists of an equimolar mixture of vanadium pentoxide and germanium dioxide. One sample was unannealed and the other was annealed at 300° C for about 24 h. The results revealed that the increase in the electrical conductivity of the annealed samples could be attributed to the increase of reduced valence states of vanadium ions which accompany the microstructure formation and not solely to the structural change. Afzal Sheikh in electron diffraction studies is appreciated.     

Novel structural properties of the lead–vanadate–tellurate glass ceramics

In this paper, we have examined and analyzed the effects of systematic intercalation of the lead ions on vanadate–tellurate glass ceramics with interesting results. The structural properties of the lead–vanadate–tellurate glass ceramics of compositions xPbO·(100 − x)[6TeO₂·4V₂O₅], x = 0 − 100 mol%, are reported for the first time. It has been shown by X-ray diffraction that single-phase homogeneous glasses with a random network structure can be obtained in this system. Among these unconventional lead–vanadate–tellurate glass ceramics, we found that network formers are good host material for lead ions and are capable to intercalate a variety of species such as Te₂V₂ ⁵⁺O₉, Pb₃(V⁵⁺O₄)₂, Pb₂V₂ ⁵⁺O₇, and V₂O₅-rich amorphous phase. On the other hand, these glass ceramics contain V⁴⁺ and V⁵⁺ ions necessary for the electrical conduction. Based on these experimental results, we propose that the V⁴⁺=O bonds are created by two different mechanisms: the first of reduction of V⁵⁺ ions to V⁴⁺ ions and thus of creation of V⁴⁺=O bonds.     

X-ray Photoelectron Spectra and Electrical Properties of the K4.3V6O16.2 Polyvanadate

The valence state of vanadium atoms in polycrystalline K_4.3V₆O_16.2 is studied by x-ray photoelectron spectroscopy. By decomposing the V 2p _3/2 peak into Gaussian components, the relative amounts of V⁵⁺, V⁴⁺, and V³⁺ ions in the polyvanadate are evaluated before and after Ar⁺ ion milling. The top surface layer of vacuum-sintered K_4.3V₆O_16.2 is shown to be enriched in potassium and oxygen ions. The 288-K dielectric permittivity, carrier mobility, and carrier concentration are determined by complex-impedance measurements.     

Effect of nanocrystallization on the structural and electrical conductivity enhancement of vanadium-based glasses

A new glass–ceramic nanocomposites material was prepared by a thermal nanocrystallization of V₂O₅–Bi₂O₃–P₂O₅ system with different V₂O₅ content. The amorphous state of glassy materials is confirmed by X-ray diffraction. It was shown by XRD and SEM studies that by suitable heat-treatment glasses can be turned into glass–ceramic nanocomposites consisting of crystallites smaller than 80 nm inserted in the glassy matrix. Also, it was shown that thermal nanocrystallization of as-prepared glassy samples leads to creation of nanocrystalline grains of V₂O₅, Bi₂O₃, and BiVO₄ phases. The glass–ceramic nanocomposites obtained show giant enhancement of electrical conductivity than the as-prepared glasses. The conductivity enhancement was recognized to interfacial regions adjacent crystalline grains. The conduction of the present glasses and their glass–ceramic nanocomposites was confirmed to be due to primarily non-adiabatic hopping of small polaron between vanadium ions.     

Structural properties of lead vanadate glasses containing La3+ or Fe3+ ions

Structural properties of lead vanadate glasses containing La³⁺ or Fe³⁺ ions were investigated using X-ray diffraction, Fourier transform infrared spectroscopy and laser Raman spectroscopy. Crystalline Pb₂V₂O₇ was formed for the molar composition 66.7PbO-33.3V₂O₅. Incorporation of greater quantities of La³⁺ into lead metavanadate glass caused the crystallization of Pb₂V₂O₇. Fourier transform infrared and laser Raman spectra also suggested the presence of LaVO₄. Incorporation of Fe³⁺ ions into lead metavanadate glass, up to 20 wt% Fe₂O₃, did not cause crystallization inside the glass matrix. Changes in the vibrational spectra are discussed.     

Structural and electrochemical behaviour of sputtered vanadium oxide films: oxygen non-stoichiometry and lithium ion sequestration

Structural and electrochemical aspects of vanadium oxide films recently reported from ICMCB/ ENSCPB have been examined using appropriate structural models. It is shown that amorphous films are nonstoichiometric as a result of pre-deposition decomposition of V₂O₅. It is proposed that the structure of amorphous films corresponds to a nanotextured mosaic of V₂O₅ and V₂O₄ regions. Lithium intercalation into these regions is considered to occur sequentially and determined by differences in group electronegativities. Open circuit voltages (OCV) have been calculated for various stoichiometric levels of lithiation using available thermodynamic data with approximate corrections. Sequestration of lithium observed in experiments is shown to be an interfacial phenomenon. X-ray photoelectron spectroscopic observation of the formation of V³⁺ even when V⁵⁺ has not been completely reduced to V⁴⁺ is shown to be entirely consistent with the proposed structural model and a consequence of initial oxygen nonstoichiometry. Based on the structural data available on V₂O₅ and its lithiated products, it is argued that the geometry of VO_n polyhedron changes from square pyramid to trigonal bipyramid to octahedron with increase of lithiation. A molecular orbital based energy band diagram is presented which suggests that lithiated vanadium oxides, Li_xV₂O₅, become metallic for high values ofx.     

Interaction of V2O5-NaY zeolite with H2S and SO2

V₂O₅-NaY zeolite catalyst was prepared by vacuum impregnation of ammonium metavanadate solution in water on NaY-zeolite (zeolite). The slurry was filtered, washed, dried, and calcined at 600°C. On heating the catalyst at 450°C in vacuum, VO²⁺ species were detected by e.s.r. It was observed that the electron-accepting and -donating sites increased when V₂O₅ or vanadium species were loaded on it. Vanadium species poisoned the active sites of the zeolite that were responsible for the formation of SO₂⁻ ions. Pretreated V₂O₅-NaY zeolite with SO₂ plays an important role in the formation of VO²⁺ species when it is treated with H₂S at room temperature. V₂O₅-NaY zeolite can be used for the reduction of SO₂ by H₂S.