Superionic Conductors in the Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript>-Bi<Subscript>2</Subscript>VO<Subscript>5.5</Subscript> System

Bi₂V_xW_{1 − x} O_{6 − y} ceramics are synthesized, and their structure and electrical properties are studied. The results indicate that the Bi₂WO₆-Bi₂VO_5.5 system contains Bi₂WO₆- and Bi₂VO_5.5-based solid solutions in the ranges 0 < x ≤ 0.2 and 0.75 ≤ x < 1, respectively. Tungsten stabilizes the high-temperature, tetragonal phase γ-Bi₂VO_5.5, which persists down to room temperature at 0.75 ≤ x ≤ 0.84. In the range 350–550°C, the electrical conductivity of the bismuth-vanadate-based solid solutions exceeds that of Bi₂VO_5.5 by about one order of magnitude. The conductivity of the Bi₂WO₆-based solid solutions is also higher than that of the host phase.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 866–870.Original Russian Text Copyright © 2005 by Voronkova, Yanovskii, Kharitonova, Rudnitskaya.     

Structure and luminescent properties of Cs<Subscript>2</Subscript>Sr(VO<Subscript>3</Subscript>)<Subscript>4</Subscript>:Mn<Superscript>2+</Superscript>

We have developed a procedure for thermally stimulated synthesis of a cesium strontium metavanadate, Cs₂Sr(VO₃)₄:Mn²⁺ (0.01, 0.50, 1.00, 5.00 at % Mn²⁺), using MnO-containing starting mixtures. The EPR spectrum of the material containing 0.01 at % Mn²⁺ shows a hyperfine structure due to the incorporation of a small amount of manganese into the diamagnetic double metavanadate host. The luminescent and optical properties of Cs₂Sr(VO₃)₄:Mn²⁺ depend on manganese content. In contrast to higher doping levels, doping with 0.01 at % Mn²⁺ increases the integrated emission intensity of the vanadate by 10% and improves its chromaticity characteristics (approaching them to those of white light). We assume that this is due to the reduction in the density of vacancy-type growth defects, such as oxygen vacancies.     

Low temperature sintering and dielectric properties of Ba<Subscript>3</Subscript>(VO<Subscript>4</Subscript>)<Subscript>2</Subscript> microwave ceramics using Co<Subscript>2</Subscript>O<Subscript>3</Subscript> additives

The Ba₃(VO₄)₂–x wt% Co₂O₃ (x?=?0.5–5) ceramics were prepared by the solid state reaction method in order to reduce the sintering temperature. The effects of the Co₂O₃ additions on the phase composition, microstructures, sintering characteristics and microwave dielectric properties of Ba₃(VO₄)₂ ceramics are investigated by an X-ray diffractometer, a scanning electron microscope and a network analyzer. As a result, the Q?×?f value of 54,000 GH, the ε _r of 14.6 and the τ_f value of +58.5 ppm/°C were obtained in the sample of the Ba₃(VO₄)₂–3 wt% Co₂O₃ ceramic sintered at the temperature of 925 °C, which is capable to co-fire with electrode metal of high conductivity such as Ag (961 °C). Moreover, the Q?×?f values of the sample with Co₂O₃ higher than that of 3 wt% additions decreased because of the formation of Ba₂V₂O₇ phase.     

Thermochemical and luminescent properties of RbVO<Subscript>3</Subscript>, CsVO<Subscript>3</Subscript>, and Rb<Subscript>0.5</Subscript>Cs<Subscript>0.5</Subscript>VO<Subscript>3</Subscript>

RbVO₃, CsVO₃, and Rb_0.5Cs_0.5VO₃ have been synthesized by the Pechini process. The vanadates have an orthorhombic structure (sp. gr. Pbcm), melt congruently in the range 650–530°C, and undergo a reversible phase transition in the range 520–340°C. We have determined the onset temperatures and end points of the transformations at a temperature scan rate of 3°C/min and their enthalpies, and measured the photo-, roentgeno-, and cathodoluminescence and diffuse reflectance spectra of the vanadates. The luminescence spectra each are well fitted with three pseudo-Voigt functions. CsVO₃ has the highest integrated emission intensity. The emission intensity of the Rb_0.5Cs_0.5VO₃ solid solution is lower than that of the simple vanadates because of the optical absorption around its intrinsic luminescence band. This may be due to the presence of stable vacancy-type structural defects in Rb_0.5Cs_0.5VO₃.     

Elastic properties of Li<Subscript>2</Subscript>Zn<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The complete elastic modulus matrix of Li₂Zn₂(MoO₄)₃ single crystals has been measured for the first time. The sound velocity has been measured in different directions of the crystals by a pulse-phase method. The measurement results have been used to calculate elastic moduli. The sound velocity has been calculated in the three main crystallographic planes of the crystals.     

Luminescence properties of Bi codoped and P codoped Ca<Subscript>3</Subscript>(VO<Subscript>4</Subscript>)<Subscript>2</Subscript>:Eu<Superscript>3+</Superscript>

New red emitting phosphors, Ca₃(VO₄)₂:Eu³⁺,Bi³⁺, Ca₃((P,V)O₄)₂:Eu³⁺ were synthesized by low temperature solid-state reaction and characterized by X-ray diffraction, scanning electron microscopy, photoluminescence spectra and Fourier transform infrared spectroscopy. The results show that the red emission located at about 613 nm was ascribed to ⁵ D ₀-⁷ F ₂ transition of Eu³⁺. The effect of by Bi doping and by P doping was also investigated systematically.     

Microstructure and microhardness of vanadium oxides in the range VO<Subscript>0.57</Subscript>-VO<Subscript>1.29</Subscript>

X-ray diffraction and optical microscopy data are presented which demonstrate that substoichiometric vanadium oxide (VO_0.57-VO_0.97) consists of a cubic phase with the B1 structure (sp. gr. Fm \(\bar 3\) m) and an ordered monoclinic phase of composition V₁₄O₆ (sp. gr. C2/m). The content of the latter phase decreases with increasing oxygen content. The superstoichiometric vanadium oxide VO_1.29 is shown to contain trace amounts of V₅₂O₆₄. Vickers microhardness data for nonstoichiometric vanadium oxides in the range VO_0.57-VO_1.29 show that, with increasing oxygen content, their H _V has a tendency to decrease, from 18 to 12 GPa. Their microhardness is shown for the first time to have a maximum near the stoichiometric composition VO_1.00.     

Compound-LiCo<Subscript>3/5</Subscript>Fe<Subscript>1/5</Subscript>Mn<Subscript>1/5</Subscript>VO<Subscript>4</Subscript> and dielectric characteristics

The LiCo_3/5Fe_1/5Mn_1/5VO₄ compound was successfully synthesized by solution-based chemical method. The variation of dielectric constant (ε_r) with frequency at different temperatures shows a dispersive behavior at low frequencies. Temperature dependence of ε_r at different frequencies indicates dielectric anomalies in ε_r at temperature (T_max) = 220, 235, 245, 260 and 275 °C with (ε_r)_max ~ 6,830, 2,312, 1,224, 649 and 305 for 10, 50, 100, 200 and 500 kHz, respectively. The variation of tangent loss with frequency at different temperatures shows the presence of dielectric relaxation in the material. The variation of relaxation time as a function of temperature follows the Vogel-Fulcher relation.     

Synthesis and growth of TlInSe<Subscript>2</Subscript> and TlGaSe<Subscript>2</Subscript> single crystals

Methods of synthesis and growth of TlGaSe₂ and TlInSe₂ single crystals are developed. It is shown that the Bridgman-Stockbarger technique is efficient for growth of large TlGaSe₂ single crystals, and zone recrystallization is efficient in the case of TlInSe₂.     

Anisotropy of dielectric losses in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> single crystals

The procedure and results of measurements of the dielectric loss tangent using the dielectric resonator technique on azimuthal modes of the HE (quasi-E) and EH (quasi-H) types are considered. The measurements were performed for uniaxial anisotropic single crystals of Al₂O₃ (at a frequency of 11 GHz) and SiO₂ (at 39 GHz) in a temperature range of 80–373 K and for an isotropic single crystal of Y₃Al₅O₂ (YAG) at room temperature in a frequency range of 9–15 GHz. The proposed method revealed the anisotropy of dielectric losses in Al₂O₃ and SiO₂ single crystals in the temperature range studied. According to this, losses along the optical axis of these crystals are lower than in the transverse plane. In the YAG crystal, the Q values for modes of the two types with the same frequency are close, which corresponds to isotropic losses. The dielectric losses in YAG increase in proportion to the frequency.     

Radiative recombination in CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

We have studied the effects of light polarization and temperature on the photoluminescence spectrum of CdGa₂S₄ single crystals at temperatures from 77 to 300 K. Based on analysis of the present experimental data in conjunction with earlier band structure, optical absorption, and photoconductivity data, we have identified the mechanism of radiative recombination in CdGa₂S₄.     

Photoconductivity and luminescence anisotropy in ZnIn<Subscript>2</Subscript>S<Subscript>4</Subscript> and ZnIn<Subscript>2</Subscript>S<Subscript>4</Subscript>:Cu<Superscript>2+</Superscript> single crystals

ZnIn₂S₄ and ZnIn₂S₄:Cu²⁺ single crystals are shown to exhibit considerable photoconductivity and luminescence anisotropy. The mechanism of carrier recombination in the crystals is polarization-dependent. Their spectra can be understood in terms of optical transitions involving donor and acceptor levels. The degree and spectral distribution of the anisotropy can be tuned via doping and alternating growth of the three- and one-layer polytypes.     

Synthesis and characterization of new glasses based on Li<Subscript>2</Subscript>S-P<Subscript>2</Subscript>S<Subscript>5</Subscript>-Sb<Subscript>2</Subscript>S<Subscript>3</Subscript> system

There is great interest in sulfide glasses because of their high lithium ion conductivity. New sulfide glasses based on Li₂S-P₂S₅-Sb₂S₃ system have been synthesized by a classical quenching technique. A summary of thermal and structural characterization is presented. Electrical conductivities of the samples have been determined by Impedance Spectroscopy. The compositions of low lithium content (below 20% mol) have presented low electronic conductivities close to 10⁻⁸ S/cm at room temperature. The compositions of medium lithium content (30–50% mol) have presented mixed ionic-electronic behavior with predominant on ionic conductivity with a maximum values around 10⁻⁶ S/cm for samples up to 50% Li₂S at room temperature. Arrhenius behavior is verified between 25°C and T_g for all glasses with activation energies about 0.55–0.64 eV. A comparative study of conductivities with glasses belonging to the other chalcogenide systems has been undertaken.     

Acoustophotovoltaic effect in TlIn<Subscript>0.98</Subscript>Gd<Subscript>0.02</Subscript>Se<Subscript>2</Subscript> single crystals

An acoustophotovoltaic effect in TlIn_0.98Gd_0.02Se₂ single crystals has been detected and investigated.     

Extruded thermoelectric materials based on Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> solid solutions

Extruded n-type materials based on Bi₂Te₃-Bi₂Se₃ alloys containing 6 to 40 mol % Bi₂Se₃ have been investigated using microstructural analysis and thermoelectric measurements at room temperature and in the range 100–400 K. Their electrical properties have been compared to those of single-crystal analogs. Compositions have been found at which the extruded materials offer the highest thermoelectric performance in different temperature ranges.     

Novel synthesis and electrochemical properties of Mn(VO<Subscript>3</Subscript>)<Subscript>2</Subscript> as a high capacity electrode material in lithium-ions batteries

Mn(VO₃) powders was successfully synthesized by an effective and simple route-rheological phase reaction method and investigated as a cathode material for lithium-ion batteries. The structures, morphologies, phase purity of the prepared powder were characterized by X-ray diffraction, transmission electron microscope and X-ray photo-electron spectrometry, respectively. The results showed that the different heat temperatures could influence the particle size and crystallinity of the product. The charge-discharge experiments were performed to investigate the electrochemical properties of Mn(VO₃)₂ powder at a constant current density of 1.0 mA/cm² in a potential range of 0.0 and 3.5 V. The discharge capacity (441.1 mAh/g) showed only 31.7% losses of the initial discharge capacity (645.9 mAh/g) after 40 cycles.     

Low temperature synthesis and photoluminescence of Ca<Subscript>3</Subscript>(VO<Subscript>4</Subscript>)<Subscript>2</Subscript>:Eu<Superscript>3+</Superscript> nanocrystals with Li<Superscript>+</Superscript> addition

Different crystalline Ca₃(VO₄)₂ nanocrystals have been synthesized successfully via a facile low temperature method with lithium addition. After different ration of Li⁺ doping into the Ca₃(VO₄)₂: Eu³⁺ host, the crystallinity of the sample becomes different, resulting in different of luminescence intensity of the characteristic emission of Eu³⁺ ions. This approach provides economically viable route for large-scale synthesis of this kind of nanomaterials.     

In situ nanomechanical behaviour of coexisting insulating and metallic domains in VO<Subscript>2</Subscript> microbeams

Measuring the electrical and mechanical responses of coexisting phases at nanoscale provides a platform to engineer micro-/nanoscale pattern of metallic and insulating domains with control over properties to make novel devices. Here, we employ several in situ characterization techniques, namely Raman, optical imaging and electrical measurements, to identify the phase coexistence of metallic and insulating domains. Further, we performed site-specific in situ nanoindentation to address the spatial variation in nanomechanical properties in vanadium dioxide (VO₂) single-crystal microbeams in proximity to metal–insulator transition temperature. We also investigated load or contact depth dependence on elastic modulus at various temperatures to avoid the interference of indentation size effect on nanomechanical properties across the phase transition. The obtained results confirm the abrupt increase in elastic modulus (~17 GPa) and nanohardness (1 GPa) across the transition from monoclinic (insulator) to rutile (metal) phase.     

Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.     

Synthesis and crystal structure of [UO<Subscript>2</Subscript>(OH)(CO(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>)<Subscript>3</Subscript>]<Subscript>2</Subscript>(ClO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The complex [UO₂(OH)(CO(NH₂)₂)₃]₂(ClO₄)₂ (I) was synthesized. A single crystal X-ray diffraction study showed that compound I crystallizes in the triclinic system with the unit cell parameters a = 7.1410(2), b = 10.1097(2), c = 11.0240(4) Å, α = 104.648(1)°, β = 103.088(1)°, γ = 108.549(1)°, space group \(P\bar 1\), Z = 1, R = 0.0193. The uranium-containing structural units of the crystals are binuclear groups [UO₂(OH)· (CO(NH₂)₂)₃] ₂ ²⁺ belonging to crystal-chemical group AM²M ₃ ¹ [A = UO ₂ ²⁺ , M² = OH^?, M¹ = CO(NH₂)₂] of uranyl complexes. The crystal-chemical analysis of nonvalent interactions using the method of molecular Voronoi-Dirichlet polyhedra was performed, and the IR spectra of crystals of I were analyzed.