M. Nevřiva

On the determination of the CuO-BaCuO2 and CuO-YCuO2.5 binary phase diagrams

Abstract The phase relations in the CuO-BaCuO 2 and CuO-YCuO 2.5 systems were investigated by means of DTA and X-ray diffraction analysis. Decomposition of CuO into Cu 2 O and mutual reaction of the melt containing BaO complicated the phase diagrams determination.

Charge distribution, pressure and composition effects of CuOx based superconductors

Abstract Charge distribution on copper and associated forms of oxygen anions are discussed. Pressure effects on Tc are surveyed for various YBCO, BSCO and TBCO superconductors. Pseudobinary and quasiternary YO1.5-BaO-CuOx phase diagrams are reviewed including pseudobinary cuts between YBa2Cu3Ox, Y2BaCuO5, BaCuO2 and YCuO2.5.

Heat capacity of CuO in the temperature range of 298.15–1300 K

The isobaric heat capacity of CuO was measured in the temperature range of 350‐1130 K by DSC. The temperature dependence of the heat capacity exhibits rather unusual course which can be explained by significant magnetic contribution extending up to’900 K. The derived heat capacity polynomial equation fits the published experimental heat content data up to 1300 K with sufficient accuracy. # 2000 Elsevier Science B.V. All rights reserved.

Solid-liquid equilibria and crystallization in the Bi-Sr-Cu-(O) system

Abstract The study of phase relationships in a part of the Bi-Sr-Cu-(O) system was carried out in order to determine approximate boundaries of liquid, liquid+solid and two solids+liquid regions at 800°C and for 850°C, respectively, in an area where Bi2Sr2CuO6 and Bi2+xSr2-yCuO6+y phases occur. In particular, attention was paid to the delimitation of the primary crystallization field of the latter phase with regard to its importance for the crystal growth.

On phases and characterisation of quenched YBa2Cu3Ox

Abstract It is shown that quenching of the YBa 2 Cu 3 O x melt at different cooling rates yields (besides the known phases of Y 2 O 3 , CuO, BaCuO 2 and Y 2 BaCuO 5 ) some unidentified phase(s) other than YBa 2 Cu 3 O x . YBa 4 Cu x O x and YBa x Cu 3 O x . Upon reheating the as-quenched samples oxidation and precipitation exotherms are revealed between 250 and 500 °C. The as-quenched and annealed (at 300 and 500 °C) samples do not show superconductivity. This is not achieved unless the sample is heated at temperatures which enable solid-state reactions to occur between the matrix components to form YBa 2 Cu 3 O x .

Preparation and the crystal structure of a new manganate, Sr4Mn3O10

Abstract The growth of strontium manganates from melts based on SrCl2SrF2 as solvents yielded orthorhombic crystals of composition Sr4Mn2.964Pt0.036O10 (space group Cmca, a = 5.443(2), b = 12.427(4), c = 12.500(4) A, Z = 4, ϱcal = 5.34 g cm−3). The platinum incorporated in samples originates from the crucible and may be essential for the crystal stabilization. The structure can be derived from a close-packed arrangement of oxygen and strontium atoms, in which some positions are vacant. This results in a more open structure. Manganese and platinum cations occupy all octahedral voids coordinated by the oxygen atoms only. The characteristic feature of the structure is triple groups of face-shared octahedra which are interconnected by common corners into a two-dimensional framework perpendicular to the [010] direction. Stacking in the third dimension is established by SrO bonds.

Phase diagram of the pseudobinary BaCuO2-YBa2Cu3O6.5-YCuO2.5 system

Abstract The recent discovery of high temperature superconductors has led to the study of the phase equilibria in the corresponding systems, particularly Y-Ba-Cu-O. Obviously the investigation of pseudobinary regions is a necessity for a complete understanding of such a complex system. In this work the YCuO 2.5 -BaCuO 2 tie line, including the superconductive 1:2:3 phase, in the temperature range 900–1300 ° C at P O 2 = 100 kPa was investigated using X-ray phase analysis and DTA. Samples were heated in an oxygen atmosphere at various temperatures and were subsequently quenched. The resulting pseudobinary phase diagram of the YCuO 2.5 -BaCuO 2 system was constructed assuming that copper ions were present as Cu 2+ ; deviations from stoichiometry were neglected.

Growth and characterization of Y−yPryBa2Cu3O7−x single crystals

Abstract Starting from the recently reported isothermal section of the YBaCu(O) system in air at 950°C the optimum compositional region allowing the growth from a homogeneous melt and a replacement of yttrium for praseodymium in Y − y Pr y Ba 2 Cu 3 O 7− x single crystals was determined. Besides that the effects of crucibles during the growth and of a post-annealing modification of the oxygen stoichiometry on the resulting electric properties are reported.

Thermochemical properties of MeCuO2 and Me2CuO3 (Me=Ca, Sr, Ba) mixed oxides

Abstract Optimized experimental data and estimated values are utilized for the assessment of thermochemical functions of alkali-earth cuprates. For the determination of temperature dependences of heat capacities the Neumann–Kopp additive rule was employed. Values of standard enthalpy of formation and molar entropy of Ca 2 CuO 3 , SrCuO 2 , Sr 2 CuO 3 and BaCuO 2 were optimized from experimental results. For the estimation of enthalpies of formation of CaCuO 2 and Ba 2 CuO 3 a linear correlation between enthalpy of formation from oxides and ionic radius of alkali-earth cations was used. Molar entropy of CaCuO 2 and Ba 2 CuO 3 was estimated using the modified contribution method.

Superconductivity and electric transport of Bi2+xSr2−yCuO6+δ single crystals

A series of Bi2+xSr2−yCuO6+δ single crystals in the compositional range 0.11 < x < 0.4 and x = y was investigated by means of resistivity, thermopower and Hall effect measurements. In order to elucidate the influence of the oxygen stoichiometry the studies were carried out successively in the as-grown, oxygen and argon post-annealed crystals. The superconductivity appears in an extremely narrow composition range, 0.11 < x < 0.15. This fact is discussed in the context of the doping level and charge carrier localization ascribed to bismuth ions which occupy the Sr sites partially.