Identification of superconducting phases in the Ba-Ca-Cu-O system: an unstable phase with Tc≈126 K and its derivative with Tc≈90 K

Abstract

Superconducting samples with to line T  _c values as high as 126 K were obtained in the Ba-Ca-Cu-O system by means of high-pressure synthesis. The X-ray powder diffraction pattern obtained for the as-synthesized 126 K phase was indexed with a tetragonal unit cell and lattice parameters, a=3.85 Å and c=28.3 Å. When exposed to air, the 126 K phase changes to another superconducting phase with T  _c ≈90 K. The crystal structure of the 90 K derivative phase was successfully studied by electron and synchrotron X-ray powder diffraction. The unit cell has body-centered symmetry, space group I4/mmm, a=3.8486 Å, c=33.979 Å. The metal composition in the 90 K phase was estimated by EDX to be Ba:Ca:Cu=2.0:2.1:3, and from TG-MS and IR spectroscopy analyses the presence of H ₂ O molecules (and possibly also carbonate groups) was revealed. A layered-cuprate M-m223 structure was concluded for each of the two phases. For the 90 K phase, two types of structural models, assuming either H or (H,C) composition for the charge-reservoir constituent M, were derived and tested. However, refinement of the synchrotron diffraction data could not differentiate between the two models, both giving reasonable agreement with experimental data. For the 126 K phase, a 0223 structure is proposed, and the location of the oxygen atom in the (BaO) ₂ double layer is discussed. Finally, it is believed that, upon the phase change, besides H ₂ O (CO ₂ ) molecules, additional protons from a redox reaction involving copper and/or peroxide-type oxygen are incorporated into the 126 K phase.