Structures of superconducting Ba2YCu3O7-ϖ and semiconducting Ba2YCu3O6 between 25°C and 750°C

The structure and copper valence states of the 100K superconductor, Ba₂YCu₃O₇ have recently been determined by neutron powder diffraction between 5K and 300K¹, and at room temperature². This ′oxygen deficient orthorhombic perovskite structure′, now independently confirmed^3,4, is well ordered and stochiometric, and contains one Cu⁺⁺⁺ and two Cu⁺⁺ atoms distributed over two square planar oxygen co-ordinated sites. In the a-b plane CuO₄ squares are linked by their oxygen corners to form infinite sheets, while along the b-axis CuO₄ squares form infinite chains on the second Cu-site. In this paper we show that when Ba₂YCu₃O₇ is heated above room temperature, it progressively loses all oxygen from the O4 sites on the b-axis chains, while the remaining oxygen sites remain fully occupied. Near 700°C it becomes tetragonal⁵, with the P4/mmm Ba₂YCu₃O₆ structure obtained by X-ray diffraction at room temperature⁶. This material is well ordered, stochiometric and stable when cooled under vacuum. The valence of Cu2 in the a-b sheets remains close to 2, while that of Cu1 in the b-axis chains falls to near 1. The valence of copper in these chains can perhaps fluctuate, since the vibration of O4 out of the chain, which we observed even at 5K¹, implies a reduction of the valence charge on this copper site. We have observed similar large amplitude oxygen vibration at low temperature in La₂CuO₄.     

Specific Heat of the Er2Ge2O7–Er2Sn2O7 Solid Solutions in the Temperature Range of 350–1000 K

Physics of the Solid State - Er2Ge2O7–Er2Sn2O7 solid solutions have been obtained using solid-state synthesis by burning the stoichiometric mixtures of the initial oxides in air in the...     

Kinetics and Phase Transformations during the Decomposition of Supersaturated Solid Solutions of Magnesium in Mg–Dy–Sm Alloys

Kinetics and phase transformations are studied for the previously unconsidered decomposition of supersaturated solid solution in magnesium-based alloys Mg–Dy–Sm. The decomposition of solid solution slows when there is an increase in % Dy: % Sm ratio in the alloys. Mg–Sm and Mg–Dy alloys grow harder when Dy and Sm are added to them, respectively. Products of decomposition in ternary Mg–Dy–Sm alloys are the same as in binary alloys Mg–Dy and Mg–Sm but contain combinations of Dy and Sm.     

Pyroelectric and Electrocaloric Effects in PMN–PbTiO3–SrTiO3 Solid Solutions

Physics of the Solid State - The pyroelectric and electrocaloric effects and also the dielectric properties in a biasing electric field have been studied in lead–strontium...     

Influence of Nanoscale Parameters on Solid–Solid Phase Transformation in Octogen Crystal: Multiple Solution and Temperature Effect

JETP Letters - In this study, the influence of two nanoscale parameters (e.g., ratios of energy and width of two different interfaces) and temperature have been investigated which drastically...     

Ultrasonic and FT-IR studies on Bi2O3–Er2O3–PbO glasses

Glasses in the 90Bi₂O₃–(10?x)Er₂O₃?xPbO (x = 3, 5, 7, 9 and 10 mol%) system have been prepared by the melt-quenching technique. Elastic properties and FT-IR spectroscopic studies have been employed to study the role of PbO in the structure of the investigated system. Elastic properties and Debye temperature were recorded using sound wave velocity measurements at 4 MHz at room temperature. The results showed that density increased and molar volume decreased, while both sound velocities increased with an increase in x. Infrared spectra of the glasses revealed that the bismuthate network is affected by an increase in PbO content. The results are interpreted in terms of the conversion of [BiO₆] into [BiO₃] structural units, indicating that Pb ions have been substituted for erbium ions as tetrahedral network formers. The elastic moduli increased with increasing PbO content due to the increased average bond strength and degree of connectivity, as a direct effect of the increase in [BiO₃] structural units.     

Thermal Conductivity of Single-Crystal ZrO2-Based Solid Solutions Stabilized with Scandium and Yttrium Oxides in the Temperature Range 50–300 K

Physics of the Solid State - The thermal conductivity of single-crystal (ZrO2)1 – x(Sc2O3)x (x = 0.09, 0.10) and...     

Phase Transformations and Photoluminescence Features of Gd2(1 – x)TbxEuxO3 Solid Solutions

Physics of the Solid State - Solid solutions Gd2(1 – x)TbxEuxO3 (where x = 1 and 2.5 mol %) are prepared by the sol–gel method with subsequent annealing at...     

Anomalies in a Lithium Niobate Stoichiometric Crystal in the Temperature Range 100–400 K

Optics and Spectroscopy - We show that the differences in the anomalous temperature behaviors of thermoluminescence and line intensities in the Raman spectra of congruent and stoichiometric lithium...     

On the Calculation of the Debye Temperature and Crystal–Liquid Phase Transition Temperature of a Binary Substitution Alloy

A method of estimating the interatomic pair interaction potential parameters for a binary substitution alloy with consideration for the deviation of its lattice parameter from the Vegard law is proposed. This method is used as a basis to calculate the Debye temperature and Grüneisen parameters of a SiGe alloy. It is shown that all these function nonlinearly variate with a change in the germanium concentration. Based on this technique and Lindemann's melting criterion, a method for calculating the liquidus and solidus temperatures of a disordered substitution alloy is proposed. The method is tested on the SiGe alloy and demonstrates good agreement with experimental data. It is shown that when the size of a nanocrystal of a solid substitution solution decreases, the difference between the liquidus and solidus temperatures decreases the more, the more noticeably the nanocrystal shape is deflected from the most energetically optimal shape.

     

Manifestation of phase transformations in optical spectra of Na0.5Bi0.5TiO3 crystals between 25°C and 350°C

The pseudo dielectric function ?ε? (E) = ?ε ₁? (E) + i?ε ₂? (E) of sodium bismuth titanate Na_0.5Bi_0.5TiO₃, has been obtained in the spectral range of electronic excitations (2–9.5 eV) by spectroscopic ellipsometry using the synchrotron radiation source BESSY II. The spectrum contains a broad absorption band at 4.2–4.5 eV. The temperature dependence in the range of 25–350°C is presented by the related susceptibilities ?χ ₁? (T) and ?χ ₂? (T) at 5 eV. A clear peculiarity in the temperature behavior of ?χ ₁? (T) and ?χ ₂? (T) between 180 and 320°C has been revealed and discussed. In this range an extremum-like temperature dependence of intensity of the light reflected from the sample surface, has been revealed and can be explained by the diffraction of light on the grid of the elastic domain structure of the sample.     

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.     

Concentration Phase Transitions and Structural Disordering in the System of Solid Solutions LixNa1?xTa0.1Nb0.9O3

Concentration phase transitions and structural disordering in the system of solid solutions Li _x Na_1–x Ta_0.1Nb_0.9O₃ (x = 0–0.16) are investigated by the methods of Raman scattering, x–ray structural analysis, and electrophysical measurements. Points of disruption of the translational order in the arrangement of cations at x = 0.05–0.06, 0.09, 0.11, 0.135, and 0.14 that correspond to the boundaries of morphotropic regions and concentration structural anomalies are detected. At points x = 0.11; 0.135, and 0.14, the Raman spectrum in the region of vibrations of the cations located in octahedral and cubooctahedral holes (0–400 cm^–1) consists of two wide maxima and is practically smeared into the wing of the Rayleigh line, and on the concentration dependences of the electrophysical properties one observes sharp anomalies. The oxygen ochahedrons are distorted less significantly as x changes: the corresponding lines broaden, and new lines demonstrating a symmetry change of the oxygen octahedrons appear only for x 0.14. It is shown that the system of solid solutions Li _x Na_1–x Ta_0.1Nb_0.9O₃ for x < 0.015 is characterized by the antiferroelectric type of dipole ordering, and for x 0.015 the structure of a solid solution becomes noncentrosymmetric.     

Structural and optical properties of WO3–PbO–B2O3 glass-ceramic

The WO₃–PbO–B₂O₃ glasses and glass ceramics are prepared and investigated with the help of XRD, density, molar volume, UV–visible and FTIR spectroscopy. XRD pattern reveals the glassy behavior up to 4% concentration of WO₃ and ceramic behavior of the prepared samples with concentration of WO₃ >4%. Band gap of glass samples decreases with increase in the WO₃ concentration from 0–5%. The samples with WO₃ concentration >5% do not respond to UV–visible absorption. The density and molar volume measurements show the compaction of structure of the samples, which is due to the formation of BO₄ groups. FTIR spectroscopy shows the formation of BO₄ group and W–O–W bending vibration at high concentration of WO₃.     

Broadband infrared luminescence in Yb-doped Bi2O3–GeO2 glasses

The Yb-doped Bi₂O₃–GeO₂ glasses were prepared by the conventional melt quenching technique. Near-infrared (NIR) broadband emission was found at about 1024 nm, and 1330 nm (under 785 nm excitation), and the measured fluorescent lifetime was about several hundred microseconds. The emission intensity of Yb-doped Bi₂O₃–GeO₂ glasses increased with increasing of Yb dopant in our experiments. The NIR emission should be related to Yb³⁺ and lower valence Bi ions.     

Temperature dependence of Bi2O3 structural parameters close to the α–δ phase transition

This article presents the results of in situ X-ray powder diffraction, Raman spectroscopy, and electrical impedance spectroscopy of the α-phase of Bi₂O₃, at 0.1 MPa in the temperature range below and above the α–δ-phase transition. This work demonstrated subtle nonlinear temperature variations of the cell parameters, of the hard-mode Raman shift, and of the activation energy of electrical conductivity in the temperature range about 100–120°C below the α–δ phase transition temperature T _Tr ≈ 725°C in Bi₂O₃. At T < 600°C, the linear variation of the inverse dielectric susceptibility (χ ^?1) correlates well with the hard mode frequency shift Δ(ω ²) of Raman A_1g mode as Δ(χ ^?1)/Δ(ω ²) ≈ 5.5 × 10^?7 cm². A structural model describing the mechanism of O^2? anion distribution and electric dipole disordering in the vicinity of T _Tr is discussed.     

Control of Crystallization Kinetics and Study of the Thermal, Structural and Morphological Properties of an Li2O–B2O3–Al2O3 Vitreous System

A glass matrix with nominal composition 50Li₂O·45B₂O₃·5Al₂O₃ (mol%) was synthesized, and its physical properties were investigated by differential thermal analysis (DTA), X-ray diffraction (XRD), and atomic force microscopy (AFM). The glass transition temperature T _g, the crystallization-onset temperature T _x,, the crystallization peak temperatures T _c1 and T _c2, and the fusion peak temperatures T _m1 and T _m2 were determined from at least two glass matrix phases to be approximately 382, 457, 486, 574, 761, and 787?°C, respectively, at 5?°C/min heating rate. Heat treatments at 450?°C for an increasing sequence of time intervals allowed control over the amount of crystallization. Additional information on the crystallization kinetics for the LBA glass matrix was gathered from AFM images, DTA thermograms, and XRD diffractograms. The latter technique showed that LiBO₂ (ICDD-16568) and Li₃AlB₂O₆ (ICDD-51754) phases are formed in the glass?Cceramic system. Debye?CScherrer analysis of the XRD peaks revealed a competition between the evolutions of crystal phases during heat treatment. Activation energies for crystallization, obtained from theoretical models applied to the DTA data showed that the crystallization is heterogeneous. The AFM images demonstrated that this heterogeneous crystallization starts at the surface of the LBA glass matrix and identified crystal sizes in agreement with the results of the Debye?CScherrer analysis. Our study shows that thermal and structural characterization techniques can be combined with theoretical results drawn from well-tested models to offer a unified view of crystallization in a glass?Cceramics system.     

Temperature and Concentration Dependences of the Landau–Placzek Ratio in Aqueous Solutions of γ-Picoline

Optics and Spectroscopy - Ratios of the intensities of the fine-structure components at molecular light scattering (Landau–Placzek ratios) have been investigated depending on the...