R. Horyń

Possible antiferromagnetic ordering in Y2Cu2O5. Paramagnetic behaviour of BaCuO2

Abstract The susceptibility anomaly observed at 13 K for Y 2 Cu 2 O 5 strongly suggests the occurrence of antiferromagnetic Cu spin coupling in this compound. Only paramagnetic behaviour was found for BaCuO 2 .

On the magnetic ordering in the R2Cu2O5 systems

Abstract We have investigated the magnetic properties of a large group of compounds with the general formula R2Cu2O5, where R stands either for Sc, In and Lu, or the heavy rare-earth element from Tb to Yb. All compounds investigated are isostructural and crystallize in the orthorhombic Ho2Cu2O5 type of crystal structure and show some magnetic transitions at low temperatures. These transitions are mainly associated with the spins on Cu2+ sites. However, a possible magnetic ordering of the rare-earth moments by these ternary oxides has also been discussed.

The niobium-aluminium binary system phase equilibria at 1100 °C and superconductivity of alloys

Abstract The 1100 °C isothermal section of the Nb-Al binary system has been studied. The phase boundaries at 1100 °C and improved values of the lattice parameters of the Nb-Al phases, together with their superconducting properties, are presented. The Nb 2 Al phase proved not to be a superconductor down to 4.2 K.

Structural defects in LaMnO3 phase studied by neutron diffraction

Abstract The crystal structure of air-annealed lanthanum manganite of three different La/Mn ratios (0.91:1, 1:1 and 1:0.9) was refined based on neutron powder diffraction data. Rhombohedral (space group R 3 c ) and orthorhombic (space group Pnma) structure models were taken from the literature available as starting points for the crystal structure analysis. In the course of the Rietveld refinement, several models of different site occupancy were tested. Selection of the most probable models has been based on the reliability factors obtained and on the results of sample density measurements. This selection appeared to be successful for the sample of nominal composition La0.91MnO2.961 (La/Mn=0.91:1) quenched from 1270 K. The resulting distribution model, i.e. [La0.922Mn0.013]MnO3, is characterised by an excess of 0.013 Mn-ions/formula located on the La-site. The type of structural defects existing in samples with nominal compositions of LaMnO3.250 (La/Mn=1:1) quenched from 830 K, and LaMn0.9O3.062 (La/Mn=1:0.9) quenched from 1070 K, both characterised by a supplementary cationic deficiency accompanied by a fraction of excess oxygen, remain undetermined due to large uncertainties in the precise location of the excess oxygen.

Polymorphic forms and defect structure formation within homogeneity domain of LaMnO3 phase

Abstract A detailed crystallochemical characterization of air annealed LaMnO 3+ δ samples of different La/Mn ratios is presented. This characterization comprises the temperature dependencies of the lattice parameters, the valence of manganese ( V Mn ), and sample densities. The latter are compared with densities calculated for various models of possible distribution of the structure components. The data obtained have allowed: (a) to determine temperature stabilities of previously known individual structure forms appearing within the homogeneity domain of perovskite-related LaMnO 3 phase, (b) to confirm the presence of cationic deficiency, created on La and Mn sites in quantities which depend on both, the nominal La/Mn ratio and the temperature-dependent oxidation state of Mn ions greater than 3+, (c) to provide strong evidence for the presence of some excess oxygen ions. Their presumable locations may be either at vacant positions of the metal-sublattices mentioned above or at some interstitial positions available. Similarly to the cation deficiency, quantities of the excess oxygen have occurred highly dependent on the nominal La/Mn ratio and on the oxidation state of Mn ions, the latter variable versus temperature. For La/Mn=0.91:1 stoichiometry samples, a distribution characterized by a certain fraction of Mn ions on the La site has also been determined within some temperature range. Analytical formulae describing the appropriate distribution models found within the homogeneity domain of LaMnO 3+ δ versus temperature are given.

Thermal conductivity of the Y-Ba-Cu-O tetragonal structure: phase transitions and hysteresis behaviour

The authors present results of precise measurements of the thermal conductivity lambda of a non-superconducting sample of Y-Ba-Cu-O in the temperature range 1-300 K. Heating and cooling curves of lambda (T) exhibited clear hysteresis effects between 80 and 240 K, not previously reported. The temperatures at which the thermal conductivity anomalies appear were the same or close to those at which other anomalies were observed in superconducting samples. The results can be explained qualitatively if one assumes that the 1:2:3 phase is a spinodal decomposition mixture of two phases with different stoichiometry with respect to oxygen content and that a change in lattice dimensions takes place at about 80 K.

Normal state thermopower of the high temperature superconductor YBa2Cu3O6.5+λ

The behaviour of the thermopower between 50 and 350 K has been determined for YBa2Cu3O6.5+λ. A zero value of the thermopower below 90.6 K and a flat maximum of 5.52 μV K−1 at about 100 K have been found. The thermopower shows metal like behaviour. Carrier concentration 2.4 × 1021 cm−3 and carrier mobility 2.5 cm2 V−1 s−1 at ambient temperature were determined.

Growth of single crystals of the YbMo6S8 phase and refinement of its crystal structure

Abstract Single crystals of YbMo6S8 were grown in sealed molybdenum crucibles at 1800 °C. The compound was found to melt congruently. X-ray diffraction data show that the origin site is fully occupied with stoichiometry x = 1.0. Neither delocalization nor an excess of ytterbium atoms are observed, and thus it is concluded that a static mixture of divalent and trivalent ytterbium is not possible.

Preparation of single crystals of some rare earth molybdenum chalcogenide superconducting phases

Abstract Macroscopic single crystals of the superconducting phase of HoMo 6 S 8 were obtained. A new modified method of preparation is presented. The results obtained and the technological conditions required are discussed in terms of their application to the growth of single crystals of other rare earth molybdenum sulphides.

Effect of oxygen atom disordering on Tc OF YBa2Cu3O7 − x

Abstract The effects of oxygen deficiency and temperature-induced oxygen atom disordering on the critical temperature (Tc) of YBa2Cu3O7 − x are presented. Two possible influences are proposed and discussed to explain an extraordinary behaviour of Tc, observed in the oxygen-deficient region of the phase. The first influence is the effect of oxygen disordering on the b-axis Cu-O chains, which results in a state characteristic of a set of mutually perpendicular and inequivalently occupied Cu-O chains, which is possibly more favourable for superconductivity. The second is the influence of the Cu-O planes, which exist between the yttrium and the barium-ion layers, the contribution of which on Tc may exceed that of the Cu-O chains and, starting at a certain oxygen index, it may become dominating. Estimates on the upper value of Tc for 1:2:3 type compounds (165 K) and on the limit of superconductivity in the YBa2Cu3O7 − x phase have been made. They give 6.23, if determined by the oxygen index, or 3 × 10−3 if determined by the orthorombic distortion parameter p.