Study of electrical conductivity and phase transition in Bi2O3–V2O5 system

The solid solutions of bismuth–vanadate were prepared by the conventional solid-state reaction. The sample characterization and the study of phase transition were done by using FT-IR, X-ray diffraction (XRD) and DSC measurements. AC impedance measurements proved that the oxide ion conductivity predominantly arises from the grain and grain boundary contributions as two well-defined semicircles are clearly seen along with an inclined spike. The electrical conductivity of Bi₂O₃–V₂O₅ has been studied at different temperatures for various molar ratios. The isothermal conductivity increases with an increase in the concentration of V₂O₅ due to the vacancy migration phenomenon. It has been found that the conductivity of different compositions of Bi₂O₃–V₂O₅ increases and shows a jump in the temperature range 230–260°C due to the phase transition of BiVO₄ from monoclinic scheelite type to that of tetragonal scheelite type. The endothermic peak in DSC at around 260°C reveals the phase transition, which is also confirmed by the XRD and FT-IR analysis. The XRD patterns confirmed the monoclinic structure of BiVO₄.     

Pressure-induced semiconductor–metal phase transition in Mg2Si

In situ electrical resistivity measurement of powdered Mg₂Si has been performed in a diamond anvil cell up to 25.4 GPa. At about 22.2 GPa, Mg₂Si underwent a pressure-induced semiconductor–metal phase transition that took place in the Ni₂In-type structure rather than the anti-fluorite structure predicted theoretically. The other phases (anti-fluorite and anti-cotunnite) belong to the semiconductor phase.     

Activation energy and conductivity of glasses in the P2O5–V2O5–Li2O system

The conductivity of glasses in the 50\textP_\text2 \textO_\text5 - x\textV_\text2 \textO_\text5 - ( 50 - x )\textLi_\text2 \textO50{\text{P}}_{\text{2}} {\text{O}}_{\text{5}} - x{\text{V}}_{\text{2}} {\text{O}}_{\text{5}} - \left( {50 - x} \right){\text{Li}}_{\text{2}} {\text{O}} system was studied as a function of temperature and composition. For all compositions, the conductivity variation as a function of temperature followed an Arrhenius type relationship. Isothermal variation of conductivity as a function of composition showed a minimum for a molar ratio x near 20. Probable mechanisms for decrease of conductivity with decrease of vanadium oxide concentration were explained. The minimum in room temperature was attributed to increase of V⁴⁺/V⁵⁺ with decrease of vanadium oxide in specific concentrations of vanadium oxide. Activation energy increased with decrease of V₂O₅ content. This behavior was attributed to increase of average spacing between vanadium ions.     

Microstructure and varistor properties of ZnO–V2O5–MnO2 ceramics with Ta2O5 addition

The effect of Ta₂O₅ addition on microstructure, electrical properties, and dielectric characteristics of the quaternary ZnO–V₂O₅–MnO₂ vaistor ceramics was investigated. Analysis of the microstructure indicated that the quaternary ZnO–V₂O₅–MnO₂–Ta₂O₅ ceramics consisted of mainly ZnO grain and minor secondary phases such as Zn₃(VO₄)₂, ZnV₂O₄, TaVO₅, and Ta₂O₅. As the amount of Ta₂O₅ increased, the sintered density increased from 94.8 to 97.2% of the theoretical density (5.78 g/cm³ for ZnO), whereas the average grain size decreased from 7.7 to 6.0 μm. The ceramics added with 0.05 mol% Ta₂O₅ exhibited the highest breakdown field (2715 V/cm) and the highest nonlinear coefficient (20). However, further increase caused α to abruptly decrease. The Ta₂O₅ acted as a donor due to the increase of electron concentration in accordance with the amount of Ta₂O₅. The donor concentration increased from 1.97×10¹⁸ to 3.04×10¹⁸cm^?3 with increasing the amount of Ta₂O₅ and the barrier height exhibited the maximum value (0.95 eV) at 0.05 mol% Ta₂O₅.     

High-temperature heat capacity of CdO–V2O5 oxides

Vanadates Cd₂V₂O₇ and CdV₂O₆ have been prepared from CdO и V₂O₅ by three-phase synthesis with subsequent burning at 823–1073 K and 823–853 K, respectively. The molar heat capacity of these oxide compounds has been measured by differential scanning calorimetry. The enthalpy change, the entropy change, and the reduced Gibbs energy are calculated using the experimental dependences C _p = f(T). It is shown that there is a correlation between the specific heat capacity and the composition of CdO–V₂O₅ oxide system.

     

Synthesis and properties of MoO3–V2O5–PbO glasses

Lead vanadate glasses of the system xMoO₃–50V₂O₅–(50-x)PbO (0 ≤ x ≤ 25 mol. %) were synthesized and studied by FTIR and ultrasonic spectroscopy and differential scanning calorimetry to investigate the role of MoO₃ content on their atomic structure. The elastic properties and Debye temperatures of the glasses were investigated using sound velocity measurements at 4 MHz. The activation energy for the glass transition was derived from the dependence of the glass-transition temperature (T_g ) on the heating rate. Similarly, the activation energy of the crystallization process was also determined. According to the IR analysis, the vibrations of the vanadate structural units are shifted towards higher wavenumbers on the formation of bridging oxygens. The change of density and molar volume with MoO₃ content reveals that the molybdinate units are less dense than the lead oxide units. The observed compositional dependence of the elastic moduli is interpreted in terms of the effect of MoO₃ on the coordination number of the vanadate units. A good correlation was observed between the experimentally determined elastic moduli and those computed according to the Makishima–Mackenzie model. It is assumed that MoO₃ plays the role of a glass former by increasing the activation energy for the glass transition and the activation energy for crystallization and by increasing both the thermal stability and the glass formation range of the vanadate glasses.     

Proton conductivity in sol–gel-derived P2O5–TiO2–SiO2 glasses

The sol–gel method was applied to prepare the P₂O₅–TiO₂–SiO₂ glasses with high proton conductivities and chemical stability. The glasses were prepared by the reaction of the Ti- and Si-alkoxides with PO(OCH₃)₃ or H₃PO₄, followed by heating at 600 °C. The obtained glasses were porous, the average pore diameter of which was <2 and 4 nm for glasses prepared using PO(OCH₃)₃ and H₃PO₄, respectively. The phosphorous ions, occurring as PO(OH)₃ in the TiO₂-free glass, were polymerized with one or two bridging-oxygen ions per PO₄ unit with the increasing TiO₂ content. Despite the P₂O₅–SiO₂ binary glasses exhibiting high conductivities of ∼10⁻² S/cm at room temperature, they also dissolved after immersing for 24 h in water. The chemical stability of these glasses increased significantly on the addition of TiO₂.     

Formation mechanisms and crystallographic characteristics of metastable VO2(A) nanofiber hydrothermally synthesised in V2O5–H2C2O4–H2O system

There are many similarities between VO₂(B) and VO₂(A) from crystallographic view. However, missing of VO₂(A) during the preparation of VO₂ polymorph confused many researchers. Here, the preparation of VO₂(A) was studied systemically via a hydrothermal method in V₂O₅–H₂C₂O₄–H₂O system. As a metastable phase, it can be transferred from VO₂(B) by assembling process. Usually, poly-crystal VO₂(A) nano-fibers are formed by this process. On contrast, owing to the small energy gap between meta-stable VO₂(A) and stable VO₂(R), single crystal VO₂(A) with regular shape can also be obtained by exfoliating some parts of VO₂(R) during non-equilibrium cooling process. VO₂(A) has higher phase transition temperature than stable VO₂(R). The hysteresis in this phase transition can be observed by DSC measurement and the phase transition temperature of VO₂(A) can be tuned down by tungsten doping.     

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.     

Optical properties of glasses in the Li2O–MoO3–P2O5 system

Glasses xLi₂O–(50-x)(MoO₃)₂–50P₂O₅ with x = 10, 20, 30, and 40 mol% were prepared and their optical and electrical properties were investigated. Analysis of the IR spectra revealed that the Li⁺ ions act as a glass modifier that enter the glass network by breaking up other linkages and creating non-bridging oxygens in the network. The optical absorption edge of the glasses was measured for specimens in the form of thin blown films and the optical absorption spectra of those were recorded in the range 200–800 nm. From the optical absorption edges studies, the optical band gap (E _opt) and the Urbach energy (E _e) values have been evaluated by following the available semi-empirical theories. The linear variation of (αhν)^1/2 with hν, is taken as evidence of indirect interband transitions. The E _opt values were found to decrease with increasing Li₂O content by causing increase in the number of non-bridging oxygens in network. The Urbach tail analysis gives the width of localized states between 0.48 and 0.74 eV.     

Effect of laser intensity on the semiconductor–metal transition in a doped Quantum Well

We demonstrate laser induced semiconductor–metal transition through an abrupt change in diamagnetic susceptibility of a donor at critical concentration in a GaAs/Al_xGa_1−xAs Quantum Well for finite barrier model in the effective mass approximation using variational principle. We have considered Anderson‘s localization due to the random distribution of impurities in our calculation. The nonparabolicity of the conduction band is also considered. Our results without laser field agree with the earlier theoretical results and also with the recent experimental results.     

Na2O＊B2O3＊V2O5玻璃的^51V核磁共振研究

本文应用~(51)V核磁共振研究了Na_2O·B_2O_3·V_2O_5玻璃的结构,结果表明: 1)R<0.5+K时,随着R的增加,依次形成NaV_3O_8、NaVO_3和NaVO_3-B结构。 2)R=0.5+K时,全部的V_2O_5转化为NaVO_3和NaVO_3-B结构。 3)R>0.5+K时,由于硼酸盐中非桥氧数的增加,NaVO_3-B结构逐步消失,并推测NaVO_3-B结构中的B原子为四配位结构。     

Magnetic anisotropy in 5d transition metal–porphyrin molecules

Single molecule magnets(SMMs) with large magnetic anisotropy energy(MAE) have great potential applications in magnetic recording.Using the first-principles calculations,we investigate the MAE of 5 d transition metal-porphyrin-based SMMs by using the PBE and PBE+U with different U values,respectively.The results indicate that W-P,Re-P,Os-P,and Ir-P possess the considerably large MAE among 5 d TM-P SMMs.Furthermore,the MAE of 5 d TM-P can be facilely manipulated by tensile strain.The reduction of the absolute value of MAE for Ir-P molecule caused by tensile strain makes it easier to implement the writing operation.The decreasing of the occupation number of minority-spin channels of Ir-d_(x~2-y~2) orbital leads the MAE to decrease when the tensile strain increases.     

Raman scattering study of phase transition in Lu_2O_3–Ta_2O_5

The lutetium tantalate compounds obtained from Lu2O3–Ta2O5 with a molar ratio of 0.515 : 0.485 were studied by Raman scattering and x-ray diffraction. The results of the room temperature Raman scattering indicate that the sample has a phase transition between 1830?C and 1872?C, the polycrystalline is a mixture of M-LuTaO4 and Lu3TaO7(F m3m)when it is prepared at 1830?C, and a mixture of M-LuTaO4(B112/b) and Lu3 Ta O7(Fm3m) when it is prepared at above 1872?C. The sample melts at a temperature of 2050?C. The phase transition of the sample prepared at 2050?C was also investigated by the high-temperature Raman spectra, and the result indicates that no phase transition occurs between room temperature and 1400?C, which is consistent with the results from the x-ray diffraction.     

Crystallization and conductivity of 2CaO–La2O3–5P2O5 glass–ceramic with Al2O3 addition

Al₂O₃ was added to a 2CaO–La₂O₃–5P₂O₅ metaphosphate, to replace 10% of the Ca²⁺ ions by Al³⁺, forming a phosphate with the nominal composition 1.8CaO–0.1Al₂O₃–La₂O₃–5P₂O₅. The effect of Al₂O₃ addition and heat treatment on the microstructure and conductivity of the resulting glass–ceramics was investigated by XRD, SEM, TEM, and AC impedance spectroscopy. Upon transformation from glass to glass–ceramic, conductivities increased significantly. The glasses were isochronally transformed at 700 and at 800 °C for 1 h or 5 h, in air, following heating at 3 or 10 °C/min. With Al₂O₃ addition, after a heat treatment at 700 °C, 100–300 nm nano-domains of LaP₃O₉ crystallized from the glass matrix. Annealing at 800 °C produced a further order of magnitude conductivity increase for the Al-free glass, but less so for the Al-containing glass.     

Electrical properties of V2O5B2O3 glasses

Binary semiconducting glasses of xV₂O₅·(1−x)B₂O₃ system with x ranging from 0.6 to 0.9 have been investigated to elucidate their electronic conduction. The values of conductivity and activation energy of these glasses are in good agreement with previous results on most V₂O₅-based glasses. Arguments for the small-polaron as the charge carrier in V₂O₅B₂O₃ glasses are presented.     

High-temperature heat capacity of oxides of the CuO–V2O5 system

CuV₂O₆ and Cu₂V₂O₇ compounds have been produced from initial components CuO and V₂O₅ by solid-phase synthesis. The high-temperature heat capacity of the oxide compounds has been measured using differential scanning calorimetry. The thermodynamic properties (the enthalpy change, the entropy change, and the reduced Gibbs energy) have been calculated using experimental dependences C _P = f(T). It is found that there is a correlation between the specific heat capacity and the composition of oxides of the CuO–V₂O₅ system.