Hyperfine interaction of d-electrons in the amorphous semiconductors MoO3-TeO2 and MoO3-P2O5

ESR spectra of the amorphous semiconducting systems MoO₃-TeO₂ and MoO₃-P₂O₅ are presented. The occurence of a hyperfine splitting is confirmed by numerical simulations of the observed spectra and by enriching with the isotope Mo⁹⁵. The isotropic hyperfine field at the molybdenum nucleus amounts to (113±10) kG at high MoO₃ concentration. This value is compared with results of measurements on crystalline MoO₃.     

Motional narrowing in the electron-spin-resonance-spectrum of pentavalent vanadium-compounds

The activation energy of the hopping frequency of the vanadium-3d ¹-electron is derived from the temperature dependence of the ESR linewidth in the following semiconducting, pentavalent vanadium-compounds: Na_xV₂O₅, Li_xV₂O₅ (x <0.02), V₂O₅ weakly doped with WO₃ or MoO₃ (E _a ≈0.07 eV). In V₂MoO₈, vanadium-3d ₁-electrons are responsible for the electronic conductivity, too.     

Optical absorption and thermoluminescence properties of ZnF2-MO-TeO2 (MO=As2O3, Bi2O3 and P2O5) glasses doped with chromium ions

Optical absorption, thermoluminescence, infrared spectra and differential thermal analysis of three different tellurite glass systems viz., ZnF₂-As₂O₃-TeO₂, ZnF₂-Bi₂O₃-TeO₂ and ZnF₂-P₂O₅-TeO₂ containing 0.4% of Cr₂O₃, have been investigated. Results have been analysed in the light of different oxidation states of chromium ion and the most suitable host for lasing Cr³⁺ ions has been identified and reported.     

Electrochemical and structural properties of MoO3–V2O5 nanocomposite thin film electrodes for lithium rechargeable batteries

We have investigated the electrochemical properties of V₂O₅-based thin film electrodes as a function of the amount of MoO₃ by means of X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and transmission electron microscopy (TEM). XRD results show that the V₂O₅-based thin film electrodes give an amorphous characteristic. XPS results reveal the formation of V₂O₅ and MoO₃ phases. TEM results show that MoO₃ dots (5–30 nm in size) are embedded in the amorphous V₂O₅ matrix. It is further shown that cells fabricated with the MoO₃–V₂O₅ nanocomposite thin film electrodes give better cycling performance than those made with the single V₂O₅ thin film electrodes. A possible explanation for the MoO₃ nano-dot dependence on the cycling performance of the V₂O₅-based thin film electrodes is described.     

Spectroscopy of mixed molybdenum-vanadium oxides and catalytic oxidation of toluene

The EPR, FTIR, and XRD techniques are used to examine individual and mixed vanadiummolybdenum oxides of composition (1 ? x)V₂O₅: xMoO₃ prepared by coprecipitation. The phase and structural heterogeneity of these oxides is characterized. The oxidation of toluene with air oxygen to maleic anhydride on V₂O₅ and mixed oxides involves singlet oxygen ¹O₂. A correlation between the content of V(IV) ions in the mixed catalysts and the amount desorbed ¹O₂ is observed. It is demonstrated that the oxidation of toluene causes changes in the spatial distribution of the V(IV) ions in the matrix and the phase composition of the samples.     

Thermodynamic study of lithium insertion in V6O13 and Li1+xV3O8

The enthalpies of Li⁺ insertion into two V oxides of interest as cathodes for secondary Li batteries, i.e. V₆O₁₃ and Li_1+xV₃O₈ have been directly measured by solution calorimetry. By comparing the integral molar enthalpies ΔH⁰/x with the corresponding ΔG⁰/x values, ΔS⁰/x higher than expected have been found for Li_1+xV₃O₈. This has been correlated to a reorganization of its distorted structure induced by the initial Li⁺ insertion. Two other cathode materials, having the same variation of the potential with Li⁺ content, i.e. MoO₃ and (Mo_0.3V_0.7)₂O₅, show even higher ΔS⁰/x values. The entropic values stemming from structural reorganizations add to the configurational entropies of inserted ions in determining the initial profile of E as a function of Li⁺.     

Induced crystallization and the physical properties of PbO–Sb2O3–As2O3: MoO3 glass system

PbO–Sb₂O₃–As₂O₃ glasses mixed with different concentrations of MoO₃ (ranging from 0 to 1.0 mol%) were crystallized. The samples were characterized by X-ray diffraction, scanning electron microscopy and differential thermal analysis techniques. The X-ray diffraction and the scanning electron microscopic studies have revealed the presence of Pb₅Sb₂O₈, PbSb₂O₆, SbAsO₄, Sb₂MoO₆, Sb₄Mo₁₀O₃₁, As₄Mo₃O₁₅, Pb₅Sb₄O₁₁ crystalline phases in these samples. The differential thermal analysis indicated that the surface crystallization prevails over the bulk crystallization as the concentration of the crystallizing agent is increased. The infrared (IR) spectral studies exhibit bands due to MoO₄ structural units in addition to the conventional bands due to various antimonate and arsenate structural groups. The studies on PbO–Sb₂O₃–As₂O₃: MoO₃ glass-ceramics with respect to various physical properties viz., dielectric properties over a range of frequency and temperature, optical absorption, electron spin resonance (ESR) and magnetic susceptibility at room temperature have also been reported. The optical absorption, ESR and magnetic susceptibility studies indicated that the molybdenum ions exist in Mo⁵⁺ state in addition to Mo⁶⁺ state in these samples. The redox ratio found to increase as the concentration of the MoO₃ is increased. The variations observed in all these properties with the concentration of the crystallizing agent have been analyzed in the light of different oxidation states and environment of molybdenum ions in the glass ceramic network.     

Surface modification of polycrystalline oxides (V<Subscript>2</Subscript>O<Subscript>5</Subscript>, MoO<Subscript>3</Subscript>, and WO<Subscript>3</Subscript>) irradiated with high-power ion beams

The influence of a high-power ion beam on polycrystalline oxides (V₂O₅, MoO₃, and WO₃) is investigated. Oxide irradiation with ion beams with current densities of greater than ～30 A/cm² is established to initiate changes in the color of irradiated layers and lead to surface-layer particle melting. It is demonstrated that a distinctive feature of the interaction between a high-power ion beam and V₂O₅ is the formation of surface nanosheets and nanowires whose characteristic cross-sectional size and thickness are ～1 μm and up to ～40 nm, respectively. The nanosheets are generated near emerging surface cracks if the beam current density is ～100 A/cm². Possible mechanisms of surface nanostructures formation under the action of pulsed ion beams are discussed.     

Effective cross section for γ-ray-induced displacement of atoms in some two-component glasses

The article describes a calculation of the theoretical effective cross sections for the formation of defects resulting from the 1.25- MeV -ray-induced displacement of atoms which are constituents of the glass systems Na₂O-SiO₂, K₂O-SiO₂, Na₂O-TeO₂, Cs₂O-TeO₂, V₂O₅-TeO₂, and WO₃-TeO₂.     

Theoretical studies on the local structure and spin Hamiltonian parameters for the orthorhombic Cu2+ center in LiNbO3

The local structure and spin Hamiltonian parameters (the g factors and the hyperfine structure constants) for the orthorhombic Cu²⁺ center in LiNbO₃ are theoretically studied from the perturbation formulas of these parameters for a 3d⁹ ion in an orthorhombically elongated octahedron. This center is ascribed to Cu²⁺ occupying the Nb⁵⁺ site in LiNbO₃, associated with one nearest neighbour oxygen vacancy V_O along the Z axis. The planar bond lengths are found to suffer the relative variation of about 0.16 Å by compressing and stretching the Cu²⁺–O^2? bonds along the X and Y axes, respectively, due to the Jahn–Teller effect and the charge mismatching substitution of Nb⁵⁺ by Cu²⁺. Meanwhile, the effectively positive V_O can make the central Cu²⁺ displace away from the V_O along the Z axis by about 0.3 Å. The theoretical spin Hamiltonian parameters based on the above local distortions show good agreement with the experimental data.     

Mössbauer and magnetic studies of BaVS3 and V5S8 doped with 57Fe

Iron can be easily introduced in BaVS₃ and V₅S₈. It is located at the vanadium sites and has been used as a probe to analyse by Mössbauer effect the magnetic properties of its surrounding matrix. The electronic state of iron in this matrix has also been studied. It was found that in BaVS₃, the iron is in a low spin Fe³⁺ configuration $\text{(S =}\text{1}\text{2}\text{)}$. In V₅S₈ at 4.2 K, the iron is in low spin Fe²⁺ configuration (S = 0). The rapid decrease of quadrupole splitting observed between 50 and 200 K is attributed to a thermally activated change in electronic structure. The high temperature configuration (above 200 K) seems to be neither pure low spin Fe³⁺ nor high spin Fe²⁺, but a mixture of configurations fluctuating at a rate which is faster than the characteristic time of Mössbauer measurements.     

Xanes (X-ray absorption near edge structure) of V in vanadium-iron phosphate glasses

The X-ray Absorption Near-Edge Structure (XANES) of V in vanadium-iron glasses (50P₂O₅ + (50?x)FeO + xV₂O₅) have been measured. The effective charge of V ions in glasses has been determined. At low V₂O₅ concentration (x ～ 5) only V⁴⁺ with 6-fold coordination is present on the contrary a static mixed valence state (V⁴⁺, V⁵⁺) has been found at high concentrations 20?x?50. The results explain the electron hopping conductivity effects at high V₂O₅ concentration (x ～ 50) involving V⁴⁺ ? V⁵⁺ pairs and at low V₂O₅ concentration (x?10) involving V⁴⁺ ? Fe³⁺ pairs.     

Electron spin resonance of aluminum-related color centers in α-TeO2:Al

ESR observations of aluminum-doped paratellurite (α-TeO₂:Al) single crystals after electron irradiation near room temperature indicate four radiation-induced electron-like defects with similarities to the intrinsic V_o^. center, which is also present. From measurements of angular variations and tellurium hyperfine interactions it is evident that the four centers represent a family of V_o^. centers perturbed by Al³⁺; that is, V_oAl_Te^x centers. The new centers are detected only in Al-doped crystals; however, the aluminum impurity ions appear to be too far from the V_o^. to yield ²⁷Al hyperfine interaction measurable by ESR.     

The oxidation of dodecane on a vanadium-molybdenum catalyst

The catalytic oxidation of dodecane by air oxygen on a mixed vanadium-molybdenum oxide (V₂O₅ · MoO₃, 40 mol % MoO₃) was studied over the temperature range 300–350°C. The reaction at 300–330°C occurred on oxygen vacancies with the rupture of C-H bonds and formation of α-acid. Oxidation above 350°C occurred with the splitting of the C-C bond and formation of two and more acids. Singlet oxygen ¹O₂ generated in the oxidation of oxide catalyst lattice oxygen participated in the reaction. A possible mechanism of the process was considered.     

Ion-impact chemistry in the system titanium-oxygen (studies on bombardment-enhanced conductivity—III)

The bombardment of TiO₂, whether poly- or single crystalline, with Kr ions leads to an altered surface layer having the following characteristics. It exhibits a high electrical conductivity, has the diffraction pattern of finely polycrystalline Ti₂O₃, is on the average 110 ± 20 Å thick (for 30 keV Kr), and is indefinitely stable in air at room temperature. The formation of the layer is favored by increasing the target temperature. Formation is half complete at (6 ± 2) × 10¹⁶ ions/cm², hence at a dose substantially greater than that for the half completion of sputter equilibrium ([7 ± 2] × 10¹⁵ ions/cm²). One model which could lead to Ti₂O₃ can be excluded fairly readily: this is thermal-spike stimulated vaporization, as the relevant vapor pressures are too low. More satisfactory is a model in which, due to either preferential oxygen sputtering or internal precipitation of oxygen, Ti₂O₃ nuclei are formed and grow. The reason that the stoichiometry is precisely Ti₂O₃ can be rationalized by an argument based on surface binding energies (E_b), in the sense that E_b for TiO₂ to sputter congruently is 6.4 eV, to yield nuclei of Ti₃O₅ is 5.7, to yield nuclei of Ti₂O₃ is 5.1, and to yield TiO is 6.4. A similar rationalization holds also for impact-induced chemical changes observed or inferred with AgBr, CuO, Fe₂O₃, MoO₃, U₃O₈ and V₂O₅, except that here thermal-spike stimulated vaporization cannot be excluded.     

The thermochemistry of the hydrogen vanadium bronzes HxV2O5

The enthalpies of formation of seven hydrogen vanadium bronzes, H_xV₂O₅(0<x⩽3.77), prepared at ambient temperature by “hydrogen spillover”, have been determined by solution calorimetry. The enthalpy values obtained for their formation from H₂(g) and V₂O₅(s) at 298.15 K are (in kJ mol⁻¹): H_0.22V₂O₅, −1569.95 ± 1.75; H_0.46V₂O₅, −1589.96 ± 1.69; H_1.43V₂O₅, − 1670.53 ± 1.88; H_1.87V₂O₅, −1700.56 ± 1.58; H_2.79V₂O₅, −1744.23 ± 1.72; H_3.53V₂O₅, −1772.98 ± 2.38; H_3.77V₂O₅, −1781.10 ± 2.27. The stabilities of the compounds towards decomposition, disproportion and oxidation are discussed.     

Quadratic crystal field tensors and spin Hamiltonian tensors of Fe3+ in Li2Ge7O15 above and below the phase transition temperature

Dopant Fe³⁺ ions in tetrahedral and octahedral positions of Ge⁴⁺ in the crystal Li₂Ge₇O₁₅ were studied using EPR. Fe³⁺ substitutes for Ge⁴⁺ with a local charge compensation. The octahedral site and the tetrahedral sites significantly differ by the value of the invariant sumS(B ₄) of theB ₄ tensor of the spin Hamiltonian of Fe³⁺. The irreducible tensor products {V ₄ ? V ₄}₄ and {V ₄?V ₄}₂ theV ₄ tensor of the crystal field calculated using the point-charge model for octahedral and tetrahedral complexes provide the predominant contribution of the crystal field to theB ₄ andB ₂ tensors of the spin Hamiltonian of Fe³⁺, respectively. A comparison of the fourth-rank tensorsB ₄ of the spin Hamiltonian and {V ₄?V ₄}₄ of the crystal field determined at 300 K with those determined at 77 K supports the conclusion that the phase transition is accompanied by combined rotation of the [GeO₄] tetrahedra with the [Ge(1)O₆] octahedron almost unaltered. The spectrum lines are narrow and the variety of point defects in the vicinity of the paramagnetic impurity ions Fe³⁺, Cr³⁺ and Cu²⁺ is not detected. These facts are inconsistent with the statistically distributed model for the Li(2) atom. In specific cases at 300 K, when the wings of the two spectrum lines of theM→M+1 and theM+2 →M+3 transitions of Fe³⁺ ions belonging to one system of translationally equivalent positions overlap an extra line appears in the center between these lines. It is suggested that this effect is due to the soft phonon mode above the phase transition temperature.     

Protonic conduction and diffusion in the hydrous oxides V2O5·nH2O,Nb2O5·nH2O,Ta2O5·nH2O and CeO2·nH2O

Conductivity (ac and dc) measurements are reported for hydrous V₂O₅, Nb₂O₅, Ta₂O₅ and CeO₅ in the range 250<T/K<320. Ambient temperature conductivities increase with water content and (SEM) agglomerate size, the most highly conductive material being Ta₂O₅·3.92 H₂O (α₂₉₈=3×10⁻⁴S cm⁻¹). There is no simple composition dependence of conductivity activation energy. ¹H NMR relaxation time measurements in the range 170<t/K<330 are also reported. The data consistent with chemical exchange between a range of interparticle protonic environments, but there is no simple link with conductivities.     

Evidence of small-polaron conduction in V2O5-MoO3 solid solutions

Electrical resistivity and thermoelectric power measurements on V₂O₅-MoO₃ solid solutions with 1, 2, 5 and 10 mol.% MoO₃ are reported. The experimental data are in agreement with the prediction of the model involving non-adiabatic small polarons.     

Photoaccumulating film systems based on TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript> and TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript>:V<Subscript>2</Subscript>O<Subscript>5</Subscript> nanoheterostructures

The thin-film photocatalysts TiO₂/MoO₃ and TiO₂/MoO₃:V₂O₅ obtained by a combination of sol–gel and sintering techniques were studied using the photooxidation of probing dyes, EPR spectroscopy, X-ray diffraction analysis, and electron microscopy. It was shown that due to charge accumulation caused by UV irradiation, these photocatalysts retain their oxidative activity and ability for self-sterilization in the dark for a long time after irradiation was terminated (up to 5 h for TiO₂/MoO₃:V₂O₅).