Frequency domain measurements on Na2B4O7−V2O5 semiconducting glass

Summary Dielectric measurements on Na₂B₄O₇(99.5%)−V₂O₅(0.5%) glass system, in the frequency range 10⁻³ to 10⁴ Hz and temperature range 300 to 500 K, have been carried out. The normalized plots of complex capacitance have shown a single mechanism responsible for conduction for both volume and surface measurements with their close values of activation energies (0.67±0.03) eV and (0.64±0.03) eV, respectively. The low-frequency dispersion (LFD) behaviour has been observed to be perturbed by the presence of more than one competing process. The impedance plots have shown a parallel combination of a capacitor (C) and a resistor (R), with some contribution of a dispersive element due to charge accumulation in the vicinity of the electrodes. The values ofR andC were found to be of the same order of magnitude, for both surface and volume measurements. The observedR has shown a decrease with an increase in temperature due to an increase in mobility of Na⁺ ions, whereasC remains practicaly constant. The complex capacitance surface behaviour is dominated by volume, due to hygroscopy of this glass system.     

d 1 Electrons in amorphous semiconducting V2O5 and MoO3 compounds. (ESR measurements)

We have performed electron spin resonance experiments on binary amorphous V₂ O₅- and MoO₃ compounds (V₂O₅-TeO₂, V₂O₅-BaO, V₂O₅-PbO, V₂O₅-GeO₂, V₂O₅-As₂O₃, MoO₃-P₂O₅, MoO₃-TeO₂) and determined the parameters of the corresponding spin Hamiltonian by 3 cm-, 1.2 cm- and 8 mm band measurements. These measurements yielded values of the contribution to the mean energy difference of thed ¹ levels between different vanadium- or molybdenum sites caused by fluctuations of the crystal field.     

ESR spectra of V4+ ions in the semiconductor V3O7

ESR studies on the semiconductor V₃O₇ reveal the existence of intrinsic paramagnetic V⁴⁺ ions. Computer simulations of spectra suggest that the unpaired electron wavefunction extends over ca. 12–14 lattice sites. This partial delocalization of the wavefunction is assumed to arise from minor crystallographic distortions at low temperatures.     

Magnetic susceptibility and electron spin resonance of the semiconducting mixed crystal V2MoO8 (V2O5 · MoO3)

The transition metal lattice sites of V₂MoO₈ are occupied by vanadium- and molybdenum ions in a disordered way. Our measurements of the magnetic susceptibility and the electric conductivity show that V₂MoO₈ is paramagnetic and semiconducting. The ESR investigations prove the existence of localizedd electrons (Anderson localization). A hopping process of thesed electrons in vanadium 3d orbitals leads to an ESR line narrowing. Above 50 K a line broadening occurs which is interpreted as a shortening of the effective lifetime of the localizedd electrons.     

Ionic-electronic mixed conduction in LixV2O5

The dc conductivity of lithium vanadium bronze, Li_xV₂O₅ was measured on polycrystal prepared by solid-state reaction in the x region 0.25–0.70. Both electronic and ionic conduction was observed. The former increased with increase of lithium content and was nearly equal to the total conductivity 10^-1–10⁰ S/cm. The ionic conductivity (∽10^-4 S/cm) measured by dc four-probe technique decreased as the lithium content increased in the range 400–500°C. The apparent activation energy for ionic conduction varied from 57 kJ/mol for x of 0.25 to 82 kJ/mol for x of 0.50.     

Mössbauer spectra and electrical conductivity ofxFe2O3−(40-x)V2O5−60P2O5 glasses

Mössbauer spectra and electrical conductivities were measured for the purpose of studying on the conduction mechanism of xFe₂O₃?(40-x)V₂O₅?60P₂O₅ glasses. The ratios Fe²⁺/Fe²⁺+Fe³⁺ in xFe₂O₃?(40-x)V₂O₅?60P₂O₅ glasses were determined by Mössbauer spectroscopy. On the composition dependence of D. C. conductivities in these glasses, the minimum of log σ at each temperature was obtained at 25Fe₂O₃?15V₂O₅?60P₂O₅. The conductivities of ternary glassses in Fe rich region could be explained only by the changes of carrler (Fe²⁺) concentration and the hopping conduction between Fe²⁺ ions and Fe³⁺ ions in binary glasses. In V rich region, the saturation tendency of D. C. conductivities are observed. It was suggested to be explained by increasing of V⁴⁺ ions due to the influence of Fe ions.     

Vibrational spectra of Na4P2O7·10H2O

The IR and Raman spectra are measured and analysed for sodium pyrophosphate decahydrate. The spectra are interpreted on the basis of P₂O ₇ ⁴⁻ ion and water vibrations. The observed results fit with the features predicted for the factor goup model. The appearance of two sets of frequencies in the stretching and bending regions of water suggests the existence of two kinds of water molecules in the crystal. This is confirmed by deuterium substitution.     

Anisotropy of x-ray emission spectra and electronic structure of V2O5 single crystals

The results of investigation of x-rayVK- andVL-emission spectra orientation dependence for V₂O₅-single crystals are reported. The applicability of different options of electronic structure cluster calculations for the interpretation of x-rayVK-emission bands anisotropy in V₂O₅-single crystal is discussed.     

Lithium intercalation effects on the V2O5 (001) surface

The lithium intercalation effects on the V₂O₅(001) surface have been investigated by means of the LCAO-Hartree–Fock method using various clusters. Considerable charge donated by Li to the host is transferred preferentially to the bridging oxygens rather than to the vanadyl oxygens or metals. A partial reduction of the vanadium center near the lithium is obtained.     

Raman spectra and conduction properties of V1−xTixO2 single crystals

Raman scattering and electrical conductivity measurements have been performed on mixed single crystals of V_1?xTi_xO₂ system. The results show that the formation of homopolar bonds between vanadium ion pairs in the orthorhombic phase produces little alteration of the conduction and vibrational properties compared with the insulator-metal transition, which occurs at a slightly higher temperature.     

Long-wave optical vibrations of V2O5

The infrared reflection spectra of V₂O₅ single crystals have been measured in polarized light for E ∥ a, E ∥ b and E ∥ c. By Kramers-Kronig analysis the optical constants and the values of some LO- and TO-mode frequencies were evaluated. Two unknown lattice vibrations have been ascertained.     

Threshold switching in V2O5-ZnO-SrO-FeO glasses

Density and current–field characteristics related to switching phenomenon in zinc-doped strontium iron vanadate glasses having the formula (70 ? x)%V₂O₅–x%ZnO–10%SrO–20%FeO, have been investigated over a wide composition range [0 ≤ x ≤ 15] at different thicknesses and temperatures. The samples are found to show threshold switching behavior. The switching fields are found to increase with the increase of zinc content while they decrease when the temperature is increased. The activation energy for switching was found to increase with the increase of ZnO content. Finally, the switching results are discussed in terms of the electrothermal model depending on the samples thicknesses and power calculus.