Study on the crystal structure of the high T-c superconductor (Hg,Re)-1223

Abstract

Samples of the superconducting compound (Hg,Re)–1223 of nominal composition Hg0.82Re0.18Ba2Ca2Cu3O8+δ and different oxygen contents were studied using X-ray absorption and diffraction techniques. In the synthesis process, treatments under different ratio oxygen/argon gas mixtures of 5%, 10%, and 15% O2 resulted in samples named as Sample A, Sample B, and Sample C, respectively. It was determined by using the XANES technique that in these compounds the Re cation has valence +7. The study by EXAFS determined that the octahedral coordination is distorted and the distances of Re–O bonds are of the order of 1.85 A˚ for equatorial oxygen and 2.10 A˚ for the apical ones. The Rietveld refinement of X-ray powder diffraction data revealed that the studied samples presented segregation of two superconducting phases with space group P4/mmm and distinct lattice parameters, also confirmed by anomalous X-ray scattering. The main phase was associated to the phase (Hg,Re)–1223 with higher oxygen content and the secondary superconducting phase was associated with the undoped Hg–1223 phase. Measurements of ac magnetic susceptibility showed that the sample treated under 10% O2 atmosphere presented the best superconducting properties.