Oxide electronics

Abstract:

By using the high-pressure high-temperature technique, we succeeded in substituting in the mercury cuprate superconductors up to about 40% of mercury by gold. The samples were studied by X-ray and neutron powder diffraction, electron microscopy, energy dispersive spectroscopy. AC suceptibility measurements were also performed. The experimental data obtained for the Au substituted Hg-1223 compound indicate that the substitution leads to overdoping of the CuO2 layers, with a progressive decrease of cell parameters and Tc with increasing gold content. Taking into account these results, we used gold substitution to increase Tc of a Hg-1245 sample up to 110 K. The corresponding unsubstituted samples are normally obtained by the high-pressure high-temperature technique in an underdoped state with a Tc around 100 K.

Abstract:

The possibility of existence of a Hg1-xSxBa2CuO4+2x+δ solid solution [(Hg, S)-1201] was studied at 18 kbar and 920°C. Monophasic samples were obtained in the range 0.00 ≤ x ≤ 0.15, where the unit cell parameters were found to vary from a = 3.8685(1) Å, c = 9.4667(2) Å for x = 0.00 to a = 3.9010(1) Å and c = 9.3048(6) Å for x = 0.15. Samples with x ≥ 0.20 were polyphasic and the unit cell parameters of the [(Hg, S)-1201] phases were found to vary up to a = 3.9072(1) Å, c = 9.2654(5) Å for a nominal composition x = 0.30. The as-prepared phases with 0.00 ≤ x ≤ 0.05 were found to be nonsuperconducting due to overdoping, while superconductivity appeared at x = 0.10 (Tc,onset = 40 K). Tc increases with the amount of sulfate substitution due to the decrease of the average copper valence. For the annealed samples, the variation is opposite to that found for the as-prepared samples, Tc,onset being higher for the sulfur-free phase. Rietveld refinements of neutron powder diffraction data has determined ≈18% of the HgO2-2 dumbbells to be substituted by SO2-4 groups. In this structure the SO2-4 anion adopts an average S-O bond length that is slightly higher than those in BaSO4. © 1996 Academic Press, Inc.

Abstract:

We report the observation, in Pr0.6Sr0.4MnO3, of a low-temperature phase transition between Pnma and the monoclinic space group I2/a. The I2/a allotype has never previously been observed in the (R1-xAx)MnO3 manganese perovskite system. Furthermore, this space group symmetry is not present in the recently published phase diagram obtained at 30% Mn+4, indicating that electronic doping has a direct effect on the phase stability. The structural and magnetic parameters for the new phase, as obtained from Rietveld refinements of neutron powder diffraction data, are also reported. © 1996 Academic Press.

Abstract:

The structural phase diagram of A 1-x A′ x MnO 3 (A = La, Pr, Y, ...; A′ = Ca, Sr, Ba, ...) as a function of the average A-site ionic radius 〈r A 〉 was determined from neutron powder diffraction data. In addition to the well-known Pnma and R3̄c phases, a new orthorhombic phase, with space group Imma, was found for large 〈r A 〉 at low temperatures. Refined structural parameters are reported for La 0.7 Ba 0.3 MnO 3 at 1.6 K (Imma) and at 293 K (R3̄c). The Imma → R3̄c phase transition was studied as a function of temperature for the same La 0.7 Ba 0.3 MnO 3 sample, and determined to be first order. © 1996 Academic Press, Inc.