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.     

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.     

Mechanical properties of superhard boron subnitride B<Subscript>13</Subscript>N<Subscript>2</Subscript>

Microstructure and mechanical properties of bulk polycrystalline rhombohedral boron sub-nitride B₁₃N₂ synthesized by crystallization from the B–BN melt at 7 GPa have been systematically studied by micro- and nanoindentation, atomic force microscopy and scanning electron microscopy. The obtained data on hardness, elastic properties and fracture toughness clearly indicate that B₁₃N₂ belongs to a family of superhard phases and can be considered as a promising superabrasive or binder for cubic boron nitride.     

Superprotonic CsH<Subscript>2</Subscript>PO<Subscript>4</Subscript>-CsHSO<Subscript>4</Subscript> solid solutions

We have performed partial HSO₄⁻ substitution in CsH₂PO₄ and studied the associated structural changes and the proton conductivity of the resultant (CsH₂PO₄)_{1 − x} (CsHSO₄) _x solid solutions in the range x = 0.01–0.3. The results indicate that, at room temperature, the solid solutions are disordered. In the range x = 0.01–0.1, they are isostructural with the low-temperature phase of CsH₂PO₄ (sp. gr. P2₁/m), and their unit-cell parameters increase with x, whereas in the range x = 0.15–0.3 the solid solutions are isostructural with the high-temperature, cubic phase of CsH₂PO₄ (Pm3m), and their unit-cell parameter decreases. The conductivity of the (CsH₂PO₄)_{1 − x} (CsHSO₄) _x solid solutions with x ≤ 0.3 depends significantly on their composition and increases at low temperatures by up to four orders of magnitude, approaching that of the superionic phase of CsH₂PO₄ in the range x = 0.15–0.3 because of the hydrogen bond weakening and increased proton mobility. The conductivity of the superionic phase decreases with increasing x by no more than a factor of 1.5–2, and the superionic phase transition, which occurs at 231°C in CsH₂PO₄, shifts to lower temperatures and disappears for x ≥ 0.15. The activation energy for low-temperature conduction decreases with increasing x: from 0.9 eV in CsH₂PO₄ to 0.48 eV at x = 0.1.     

Two types of ABi<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>9</Subscript> layered perovskite-like ferroelectrics

Statistical analysis of the literature data on the structural and ferroelectric properties of ABi₂B₂O₉ layered perovskite-like compounds shows that these ferroelectrics can be divided into two distinct groups, one constituted by tetragonal and nearly tetragonal compounds, and the other by compounds with a significant orthorhombic (and possibly monoclinic) distortion. Surprisingly, there are no layered ABi₂B₂O₉ compounds in a wide intermediate range of structural parameters.     

Anharmonicity in the V<Subscript>2</Subscript>O<Subscript>3</Subscript> molecule and thermodynamic properties of V<Subscript>2</Subscript>O<Subscript>3</Subscript> in the gas phase

A quantum-mechanical calculation of the relative stability, structural parameters, and vibrational frequencies of V₂O₃ molecule isomers for different spin states was carried out using the BPW91/6-311+G(d, p) method. It was shown that the isomer with the C _s structure (nonplanar VOVO rectangle with an O atom attached to it) in the X ⁵ A″ electronic state possesses the maximum stability. The energy of the C _2v symmetry structure was higher than the lowest energy by just 23 cm⁻¹. It definitely indicated the impossibility of usage of the harmonic model in order to calculate the thermodynamic functions of V2O3 (g). A model is proposed based on which the energy levels and vibrational sums of states for this type of motion were calculated for the C _s → C _2v → C _s transition coordinate. These data, as well as results obtained from quantum-mechanical calculations, were used to calculate the thermodynamic functions of V₂O₃ (g) in the temperature range of T = 100–6000 K. The calculations were performed with the five excited electronic states with energies from 1000 to 9000 cm⁻¹ taken into account. A comparison with the data calculated in the “rigid rotator-harmonic oscillator” approximation was performed.     

Studies of the EPR Parameters for Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript><Emphasis Type="Italic">Y</Emphasis>Cu<Subscript>2</Subscript>O<Subscript>8</Subscript>:Er<Superscript>3+</Superscript>

The electron paramagnetic resonance (EPR) parameters (the anisotropic g factors g _x , g _y , g _z , and the hyperfine structure constants A _x , A _y , and A _z ) of the two orthorhombic Er³⁺ centers in Bi₂Sr₂ YCu₂O₈ are theoretically studied from the perturbation formulas of these parameters for a 4f¹¹ ion in orthorhombic symmetry. In these formulas, the contributions due to the admixtures of various states are taken into account, and the orthorhombic field parameters are determined from the superposition model and the local geometry of Bi³⁺ site in Bi₂Sr₂ YCu₂O₈. The calculated EPR parameters show reasonable agreement with the observed values. The anisotropy g _z >g _x (g _y ) for the g factors may be attributed to the compression of the ligand octahedra in the Er³⁺ centers.     

Luminescence of europium-doped BaO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

We have prepared europium-doped BaO-Bi2O₃-B2O₃ glasses and investigated the doping effect on the main physicochemical properties and local structure of the glasses. Using Judd-Ofelt analysis, we calculated intensity parameters (Ω₂, Ω₄, and Ω₆), spontaneous emission probabilities, the radiative lifetime, luminescence branching factors, the quantum yield of luminescence, and the stimulated emission cross sections for ⁵ D ₀ → ⁷ F _J transitions.     

<Emphasis Type="Italic">Ab initio</Emphasis> studies on [bmim][PF<Subscript>6</Subscript>]-CO<Subscript>2</Subscript> mixture and CO<Subscript>2</Subscript> clusters

Ab initio molecular dynamics studies have been carried out on the room temperature ionic liquid, 1,n-butyl,3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) and supercritical carbon dioxide mixture at room temperature and experimental density. Partial radial distribution functions (RDF) for different sites have been computed to see the organization of CO₂ molecules around the ionic liquid. Several partial RDFs around the carbon atom of CO₂ molecule are compared to find out that the CO₂ has specific interaction with a carbon atom present in the imidazolium ring. The CO₂ is also found to be very well organized around the terminal carbon atom of the butyl chain. The partial RDFs for the oxygen atoms around oxygen and carbon atoms of the CO₂ suggests that there is very good organization of CO₂ molecules around themselves even in the [bmim][PF₆]-CO₂ mixture. The instantaneous quadrupole moment tensor has been calculated for the anion and the cation. The ensemble average of diagonal components of quadrupole moment tensor of the cation have finite values, whereas the off-diagonal components of the cation and both the diagonal and off-diagonal components of the anion have the value of zero with a large standard deviation. The CPMD studies performed on CO₂ clusters reveals the greater tendency of the clusters with more CO₂ units, to deviate from the linear geometry.     

Physicochemical simulation of fusion processes of basalt and diabase with Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> and Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> + CaO

Fusion processes of basalt and diabase with sodium carbonate and its mixture with calcium oxide are investigated by methods of physicochemical simulation. Equilibrium compositions of the Si-Al-Fe-Ca-Mg-Na system are calculated at various ratios Na₂CO₃: basalt (diabase) and (Na₂CO₃ + CaO): basalt (diabase) in the temperature range of 1270–1470 K. It is shown that, on fusion with sodium carbonate, depending on conditions, the main components of fusion products are sodium metasilicate (Na₂SiO₃) and sodium metaaluminate (NaAlO₂), magnesium orthosilicate (CaMgSiO₄), sodium ferrite(III) (NaFeO₂), iron(III) oxide (Fe₂O₃), and sodium-aluminum orthosilicate (Na₂AlSiO₄). On fusion with the mixture of sodium carbonate and calcium oxide, respectively, the products are sodium metaaluminate, calcium pyrosilicate (Ca₃Si₂O₇), calcium-magnesium orthosilicate, and calcium ferrite (CaFe₂O₄).     

Preparation,characterization and photocatalytic properties of nanoplate Bi<Subscript>2</Subscript>MoO<Subscript>6</Subscript> catalysts

Bismuth molybdate (Bi₂MoO₆) nanoplates have been successfully synthesized by a simple hydrothermal process. The nanoplates were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and IR spectroscopy. The effects of hydrothermal temperature and reaction time on the structures and morphologies of the nanoplates were investigated. On the basis of TEM observation of time series samples, a possible formation mechanism of the nanoplates was proposed. Optical absorption experiments revealed that Bi₂MoO₆ nanoplates had absorption in visible-light region, but a blue shift appeared compared with the corresponding bulk materials. Photocatalytic experiments showed that the nanoplates exhibited good photocatalytic activities for degradation of N,N,N′,N′-tetraethylated rhodamine (RhB) under visible-light irradiation (λ > 420 nm).     

Superconducting Properties of Iodine-Intercalated Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10+<Emphasis Type="Italic">x</Emphasis></Subscript>

The superconducting properties of iodine-intercalated high-temperature superconducting Bi₂Sr₂Ca₂Cu₃O_10+x phase (Bi-2223) were systematically studied. It was found that for samples containing a significant amount of Bi₂Sr₂CaCu₂O_8+x , iodine intercalation results in the dramatic decrease of the inter-granular critical current density, as well as a significant decrease of the critical temperature (T _c), the critical current density in the grains (J _cg), and of the amount of Bi-2223. For samples with a large amount of Bi-2223, T _c changes insignificantly, whereas J _cg can even increase. We argue that the different behavior of the superconducting parameters is the result of various oxygen concentrations, and we explain the effect of iodine intercalation based on the parabolic dependence between T _c and the number of holes per CuO₂ layer. The H(T) curves (determined from the peak position in the loss signal of ac susceptibility) for intercalated samples deviate significantly from the quasi 2D-like behavior, pointing toward an enhancement of the 3D fluctuations of vortices. For the change in the values and dimensionality of the flux pinning in the process of the intercalation, we attempted a qualitative explanation based on the models proposed in literature.     

High-temperature heat capacity and vibrational spectra of Eu<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Eu₂Sn₂O₇ compound has been prepared by solid-state reaction (by sequentially firing a stoichiometric mixture of Eu₂O₃ and SnO₂ in air at 1273 and 1473 K) and its heat capacity has been determined by differential scanning calorimetry in the temperature range 370–1000 K. The heat capacity data have been used to evaluate the thermodynamic properties of europium stannate: enthalpy increment H°(T)–H°(370 K), entropy change S°(T)–S°(370 K), and reduced Gibbs energy Ф°(T). Raman spectra of Eu₂Sn₂O₇ polycrystals with the pyrochlore structure have been measured in the range 200–1200 cm^–1.     

Photoluminescence and Electrical Conductivity of As<Subscript>2</Subscript>S<Subscript>3</Subscript>〈Au〉 and As<Subscript>2</Subscript>S<Subscript>5</Subscript>〈Au〉 Glasses

The 77-K photoluminescence spectra of (As₂S₃)_{100 − x} Au_x and (As₂S₅)_{100 −x} Au_x (0 ≤ x ≤ 0.04) semiconducting glasses are measured for the first time. At low doping levels, the spectra of the (As₂S₅)_{100 − x} Au_x glasses are split into two components, one of which arises from the Au dopant. The temperature-dependent conductivity of the glasses shows two breaks at low Au concentrations and anomalous behavior in the range 300–360 K. Qualitative analysis of the conductivity data suggests that most of the impurity atoms have saturated valence bonds and form solid solutions with host atoms, changing the band gap of the material. A small fraction of the impurity atoms, those having unsaturated valence bonds, produce an electrically active level responsible for impurity conduction.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 876–880.Original Russian Text Copyright © 2005 by Babaev, Kamilov, Sultanov, Askhabov, Terukov.     

Electrical Conductivity of AlN-Mg<Subscript>3</Subscript>N<Subscript>2</Subscript> Solid Solutions

AlN-Mg₃N₂ solid solutions are synthesized, and their electrical conductivity and ionic transference number are measured. In the range 500–1150°C, the conductivity of the solid solutions increases with Mg₃N₂ concentration. The ionic transference number is close to unity, and the ionic transport is mainly due to aluminum ions.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 819–822.Original Russian Text Copyright © 2005 by Lesunova, Burmakin.     

Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/TiN composites produced from TiO<Subscript>2</Subscript>-Modified Si<Subscript>3</Subscript>N<Subscript>4</Subscript> powders

Si₃N₄/TiN composites have been produced by hot pressing at temperatures from 1600 to 1800°C in a nitrogen atmosphere, using silicon nitride powders prepared by self-propagating high-temperature synthesis and surface-modified with titanium dioxide nanoparticles. We examined the effect of TiO₂ content on the microstructure, phase composition, and mechanical strength of the ceramics. It is shown that titanium nitride can be formed by the reaction Si₃N₄ + TiO₂ → TiN + NO + N₂O + 3Si. The Si₃N₄/TiN composites containing 5–20% TiN have a low density, high porosity, and a bending strength of 60 MPa or lower. In Si₃N₄/TiN ceramics produced using calcium aluminates as sintering aids, the silicon nitride grains are densely packed, which ensures an increase in strength to 650 MPa.     

Thermodynamic properties of NaZr<Subscript>2</Subscript>(AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>

The heat capacity C _p ⁰ of crystalline NaZr₂(AsO₄)₃ has been measured in the range 7–650 K using precision adiabatic calorimetry and differential scanning calorimetry. The experimental data have been used to calculate the standard thermodynamic functions of the arsenate: C _p ⁰, enthalpy H ⁰(T) − H ⁰(0), entropy S ⁰(T), and Gibbs function G ⁰(T) − H ⁰(0) from T → 0 to 650 K. The standard entropy of its formation from elements is Δ_f S ⁰(NaZr₂(AsO₄)₃, cr, 298.15 K) = −1087 ± 1 J/(mol K).     

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.     

Study of structure and properties of ZnO–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Glasses of the ternary system ZnO–Bi₂O₃–P₂O₅ were prepared and studied in two compositional series 50ZnO–xBi₂O₃–(50 − x)P₂O₅ and (50 − y)ZnO–yBi₂O₃–50P₂O₅. Two distinct glass-forming regions were found in the 50ZnO–xBi₂O₃–(50 − x)P₂O₅ glass series with x = 0–10 and 20–35 mol.% Bi₂O₃. All prepared Bi₂O₃-containing glasses reveal a high chemical durability. Small additions of Bi₂O₃ (∼5 mol.%) improve thermal stability of glasses. All glasses crystallize on heating within the temperature range of 505–583 °C. Structural studies by Raman and ³¹P MAS NMR spectroscopies showed the rapid depolymerisation of phosphate chains within the first region with x = 0–15 and the presence of isolated Q⁰ phosphate units within the second region with x = 20–35. Raman studies showed that bismuth is incorporated in the glass structure in BiO₆ units and their vibrational bands were observed within the spectral region of 350–700 cm⁻¹. The evolution of properties and the spectroscopic data are both in accordance with a network former effect of Bi₂O₃.     

Local vibrational modes of N<Subscript>2</Subscript>−O<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> defects in Cz-Silicon

In this paper we investigate N₂−O _n defect complexes in Czochralski silicon (Cz-Si) by means of local density functional theory. We consider the N₂O and the N₂O₂ defect and determine their structural, electronic and vibrational properties. The calculated local vibrational modes of the N₂O defect are in good agreement with the experiment. Furthermore the calculated binding energy matches very well with the experimental estimate. Motivated by recent experimental work, where several new absorption lines in IR absorbance spectra were observed, we present first principle studies on the ground state configuration, binding energy and local vibrational modes of the N₂O₂ defect and make a tentative assignment to the experimentally observed lines at 1018 and 810 cm⁻¹.