M. L. Kovba

Inducing superconductivity and structural transformations by fluorination of reduced YBCO

Abstract Bulk superconductivity with Tc up to 94 K has been induced by fluorination of non-superconducting YBa2Cu3O6.11 using XeF2 as a fluorination agent. Strong changes on X-ray patterns were found after fluorination of reduced YBCO. High resolution electron microscopy of superconducting samples showed the presence of a new phase with c ≈ 13 A which exists as noticeable areas included within a matrix of the 123 structure or which occurs as isolated defects with a limited extension. All fluorinated compounds exhibited a strong disorder along the c-direction resulting in a ‘loss’ of c-parameter on X-ray patterns. The present results demonstrate that fluorine indeed enters the YBCO structure with a significant structural rearrangement for the high level of fluorination. The structure of the fully fluorinated YBa2Cu3O6F2 phase, possibly responsible for superconductivity, has been deduced from high resolution electron microscopy.

Synthesis and characterization of overdoped Hg-1234 and Hg-1245 phases; the universal behavior of Tc variation in the HgBa2Can−1CunO2n+2+δ series

Abstract HgBa 2 Ca 3 Cu 4 O 10+ δ and HgBa 2 Ca 4 Cu 5 O 12+ δ were prepared by high pressure/high temperature technique. The phase composition in the Hg–Ba–Ca–Cu–O system was shown to be very sensitive to synthesis conditions. A variation of the oxygen content in the starting oxide mixture by changing the BaO/BaO 2 ratio allowed the acquisition of Hg-1234 and Hg-1245 in the overdoped state. The heat treatments (nitrogen and oxygen flows, oxygen pressure 100 bar and 2000 bar) resulted in a change of T c s from initial overdoped (116 and 100 K for Hg-1234 and Hg-1245, respectively) to underdoped (95 and 85 K) and, finally, to optimally-doped states (125 and 111 K). The dependencies of T c vs. a -parameter (and, consequently, oxygen content) for these phases exhibit a cupola-shaped behavior, as it was previously found for the lower ( n ≤3) members of the HgBa 2 Ca n −1 Cu n O 2 n +2+ δ series.

Thermochemical characteristics of nonstoichiometric Ba2Cu3O5: II. Thermal transformations

Abstract Thermoanalysis (TG-DSC) of the pure compound Ba2Cu3O5+δ (023) has been carried out at oxygen pressures of 0.21 and 1 atm in the temperature range 25–970°C. It was shown that 5.5 823°C at 0.21 atm O2; > 890°C at 1 atm O2) appeared. The liquid phase transition took place at temperatures higher than 860°C (0.21 atm) and 910°C (1 atm).

Heat capacity of LixNi2 − xO2 solid solutions

We have synthesized LixNi2 − xO2 oxides in the range x = 0.1–0.84 and showed that the solid-solution system contains a two-phase region. The heat capacity of LixNi2 − xO2 has been determined by differential scanning calorimetry.

Effect of fluorination on the structure and superconducting properties of Y2Ba4Cu7O14+δ phases

Abstract Strongly reduced Y 2 Ba 4 Cu 7 O 14.09 has been successfully fluorinated using XeF 2 as a fluorination agent. The structure of the fluorinated material has been studied by a combination of X-ray diffraction, electron diffraction, high resolution electron microscopy (HREM) and X-ray microanalysis (EDX). For a high level of fluorination, the intercalation of fluorine is accompanied by a significant structural rearrangement and leads to an increase of T c from 30 to 62 K. HREM reveals the presence of a partially fluorinated, well ordered Y 2 Ba 4 Cu 7 O 14 F δ phase with c =51.96 A. Fluorine enters into anion-deficient Cu1 layers, resulting in the formation of an octahedral arrangement of part of the Cu1 atoms with a significant elongation of the apical Cu–O distances and the c -parameter. Local ordering of fluorine atoms leads to the formation of defects with a limited extension where all Cu1 atoms have an octahedral coordination; the c -parameter in these areas is as high as 53.5 A.

Thermal analysis of mercury superconductor HgBa2CuO4 + x and its precursor Ba2CuO3 + y☆

The superconducting compound HgBa2CuO4 + x and its precursor Ba2CuO3 + y have been studied in an oxygen atmosphere by TG-DTA in the temperature interval 20–1100°C. It was supposed that the succession of chemical processes taking place during the heating (20° min−1, 1 atm O2) of HgBa2Cu1.05O4.12 is as follows: It was supposed that the Ba2CuO3.23 compound during heating (1° min−1, 0.21 atm O2) and cooling (20° min−1, 0.21 atm O2) undergoes following stages:

Thermodynamic properties of nonsuperconducting Ln2CuO4 (Ln = Nd, Sm, Eu), Ho2Cu2O5 and Ln2BaCuO5 (Ln = Nd, Sm, Eu, Ho, Yb) cuprates

Thermodynamic properties of rare-earth Ln2CuO4 (Ln = Nd, Sm, Eu), Ho2Cu2O5 and Ln2BaCuO5 (Ln = Nd, Sm, Eu, Ho, Yb) cuprates were determined using the electromotive force method with a fluorine-ion electrolyte in the temperature interval of 1000–1230 K. The thermodynamic data obtained by different experimental methods were compared.

Thermochemical properties of lithium cobaltate

Samples of low- and high-temperature modifications of LiCoO2 were produced and described. The heat capacity was measured by DSC in the temperature ranges of 140–570 and 180–570 K, respectively. The enthalpy and entropy of the cobaltates with respect to temperature were calculated from the data of heat capacity.

The thermodynamic properties of Ln2BaO4 (Ln = Sm, Dy, Ho)

The thermodynamic properties of the Ln2BaO4 phases (Ln = Dy, Ho, Sm) were studied by the electromotive force method with a fluoride electrolyte (890–1180 K), solution calorimetry in 1.07 N hydrochloric acid at 298.15 K, and differential scanning calorimetry (298–860 K). The experimental data were jointly processed, and the thermodynamic functions of the compounds over the temperature range 298–1200 K were calculated.

Synthesis, structure and properties of layered bismuthates: (Ba,K)3Bi2O7 and (Ba,K)2BiO4

Abstract The synthesis, structure and properties of the layered bismithates, (Ba,K) 2 BiO 4 and (Ba,K) 3 Bi 2 O 7 , are reported. These compounds, synthesised at low temperature in closed vessels, are the first and second members of the Ruddlesden–Popper series, Ba n +1 (Pb 1− x Bi x ) n O 3 n +1 . (Ba,K) 3 Bi 2 O 7 exhibits a semiconductor-like behaviour at low temperature. No superconducting transition is observed in any of these phases. The compounds crystallise in the I -centred unit cell, and Rietveld refinement based on X-ray diffraction data revealed the buckling of the BiO 2 plane for (Ba,K) 3 Bi 2 O 7 , which can be a reason for the absence of superconductivity. However, this cell buckling and partial ordering of the Ba- and K-cations are absent in the (Ba,K) 2 BiO 4 structure, that makes the n =1 member, containing an isolated (BiO 2 )-layer, promising for further search of superconductivity among layered bismuthates.