The effect of vanadium oxides on the crystallisation of silicate glasses

The influence of vanadium oxides as catalysts for nucleation and crystal growth in CaO-MgO-Al₂O₃-SiO₂ glasses has been investigated. The effect of varying the total vanadium content and the ratio of oxidised to reduced vanadium ions has been observed. No internal nucleation was observed, but the rate of growth of crystals of anorthite and wollastonite from the surface was increased by additions of V₂O₅ and V₂O₃. The values of the growth rates and the analysed V⁵⁺ content in both oxidised and reduced glasses suggest that V⁵⁺ ions are the most active species. Increasing concentrations of V₂O₅ in the glass gave maxima in the growth rates between 2 and 4 wt % for crystallisation temperatures between 900 and 980° C.     

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.     

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.     

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⁺).     

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.     

L'oxyde de vanadium amorphe hydrate

Hydrated amorphous vanadium pentoxide obtained by quenching of molten V₂O₅ in water is compared to amorphous V₂O₅ previously obtained by splat cooling. The first one contains some strongly bonded water which prevents crystallisation up to 300°C. Short range order around V⁴⁺ ions seems to be the same in both samples and in orthorhombic V₂O₅. Disorder in the amorphous phases leads to a localization of the charge carriers, the electron mobility being about ten times smaller than in the crystalline oxide.     

Electron spin resonance studies of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

Electron spin resonance studies of evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ amorphous thin films have been made. For lower molar contents of B₂O₃, the co-evaporated V₂O₅/B₂O₃ films show poorly resolved hyperfine structure, whereas for higher content of B₂O₃ the hyperfine spectra are well resolved. This behaviour of films is attributed to the increase in the lifetime of a particular V⁴⁺ ion due to Anderson localization of charge, as the degree of disorder increases with increase in the molar content of B₂O₃. The unpaired electron at a given time is localized on a single ⁵¹V nucleus. The low intensity of ESR signal for higher concentration of V₂O₅ in the co-evaporated V₂O₅/B₂O₃ films has been related to the less effective concentration of V⁴⁺ ions due to antiferromagnetic coupling of the V⁴⁺ ions.     

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.     

Synthesis and properties of Pb1-xVxO2-x(OH)x, solid solutions

Pb_1-xV_xO_2-x(OH)_x (0 <x ≤0.30) solid solutions with the rutile structure were prepared by reacting humid β-PbO₂ + V₂O₅ mixtures at temperatures from 200 to 250°C. At higher vanadium contents, lead hexavanadate, PbV₆O₁₆, was formed, as inferred from IR spectroscopy data. The structural and electrical properties of the solid solutions were studied, and the valence state of the constituent ions was determined. All of the vanadium has a valence of 4+ forx ≤ 0.06 and 5+ forx > 0.15. The lead ions are in the oxidation states 4+ and 2+ over the entire range of solid solution     

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.     

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.     

Structural and electrochromic properties of molybdenum doped vanadium pentoxide thin films by sol-gel and hydrothermal synthesis

Molybdenum-doped vanadium pentoxide (Mo-doped V₂O₅) thin films with doping levels of 3-10 mol% were prepared by dip-coating technique from a stable Mo-doped V₂O₅ sol synthesized by sol-gel and hydrothermal reaction. The Mo-doped V₂O₅ films had a layered V₂O₅ matrix structure along c-axis orientation with Mo⁶⁺ as substitutes. Values of the inserted and extracted charge density of 21.4 and 21.3 mC·cm^− 2 and the transmittance variation (ΔT at 640 nm) between anodic (+ 1.0 V) and cathodic (− 1.0 V) colored states of 41% were observed for the films with 5 mol% Mo⁶⁺ doping. Above this dopant concentration, the charge capacity and ΔT decreased. The enhancement of the electrochemical and electrochromic properties of the films is related to changes in the electronic properties of V₂O₅ films due to the creation of energy levels in the band gap of V₂O₅ by the Mo doping, accompanied by the reduction of the forbidden-band width and the increase of the conductivity.     

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.     

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.     

Spectrochemical studies on charge transfer bands due to d0, d5 and d10 ions in a sodium silicate glass

Titanium, vanadium and copper are normally present in glasses in their variable valency states but only Ti⁴⁺ (3d⁰), V⁵⁺ (3d⁰) and Cu⁺ (3d¹⁰) ions were found to exhibit charge transfer bands in glasses in the ultra-violet region of light. The molar extinction coefficients of these ions were calculated at their wavelength maxima in a 30Na₂O·70SiO₂ glass using Beer's-Bouger's equation and the intensities of their bands were found to lie of the order of around 10³ gm mol lit^–1cm^–1. The values of the molar extinction coefficients for these ions were compared with those of Ce⁴⁺ (5d⁰), Cr⁶⁺ (3d⁰) and Fe³⁺ (3d⁵) ions calculated earlier in 30Na₂O·70SiO₂ glass at their wavelength maxima in UV-region. The mechanism of electronic transition was suggested as L M, M L and M L M charge transfer available till date as a result of absorption of high energy UV-radiation extensively for d⁰, d⁵ and d¹⁰ ions respectively in glass depending upon the glass melting conditions. The low energy tail, of the UV-bands due to all these ions were found to obey Urbach's Rule in the present sodium silicate glass. The intensities of the charge transfer bands due to these ions are expected to depend upon their nature and symmetries, electronic configurations and wavelength of maximum absorption in the glass.     

Oxidation potential control of VO2 thin films by metal oxide co-sputtering

For metal-to-insulator transition (MIT) in vanadium oxide thin film, a thermodynamically stable vanadium dioxide (VO₂) phase is essential. In VO₂ films sputter-deposited on a quartz substrate from a V₂O₅ target, a radio-frequency (RF) magnetron sputter system at working pressure of 7 mTorr is used. Due to the lower sputtering yield of oxygen compared to vanadium leading to oxygen-ion deficiency, the reduction of V ions is resulted to compensate charge with the oxygen ions. Under lower working pressures, the deposition rate increases, but a simultaneous oxygen-ion deficiency causes the destabilization of VO₂. To prevent this, titanium oxide co-deposition is suggested to enrich the oxygen source. When TiO₂ is used, it is found that the Ti ion has a stable +4 charge state so that the use of extra oxygen in sputtering prevents the destabilization of VO₂. However, this is not the case for TiO. For the latter, Ti ions are oxidized from the +2 state to the +3 and +4 states, and V ions with less oxidation potential are reduced to +3 or so. Pure VO₂ thin film exhibits MIT at 66 °C and a large resistivity ratio of four orders of magnitude from 30 to 90 °C. The (V₂O₅ + TiO₂) system under working pressure as low as 5 mTorr yields fairly good films comparable to pure VO₂ deposited at 7 mTorr, whereas the use of TiO yields films with MIT absent or considerably weakened.     

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.     

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.     

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.     

Electrical properties of V<Subscript>2</Subscript>O<Subscript>5</Subscript>-CaO-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses exhibiting majority charge carrier reversal

The thermoelectric power and d.c electrical conductivity of x V₂O₅⋅40CaO⋅(60−x)P₂O₅ (10 ≤ x ≤ 30) glasses were measured. The Seebeck coefficient (Q) varied from +88 μ V K⁻¹ to −93 μV K⁻¹ as a function of V₂O₅ mol%. Glasses with 10 and 15 mol% V₂O₅ exhibited p-type conduction and glasses with 25 and 30 mol% V₂O₅ exhibited n-type conduction. The majority charge carrier reversal occurred at x = 20 mol% V₂O₅. The variation of Q was interpreted in terms of the variation in vanadium ion ratio (V^{5 +}/V^{4 +}). d.c electrical conduction in x V₂O₅⋅40CaO⋅(60−x)P₂O₅ (10 ≤ x ≤ 30) glasses was studied in the temperature range of 150 to 480 K. All the glass compositions exhibited a cross over from small polaron hopping (SPH) to variable range hopping (VRH) conduction mechanism. Mott parameter analysis of the low temperature data gave values for the density of states at Fermi level N (E_F) between 1.7 × 10²⁶ and 3.9 × 10²⁶ m⁻³ eV⁻¹ at 230 K and hopping distance for VRH (R_VRH) between 3.8 × 10⁻⁹m to 3.4 × 10⁻⁹ m. The disorder energy was found to vary between 0.02 and 0.03 eV. N (E_F) and R_VRH exhibit an interesting composition dependence.