Investigation of an Unstable T_c approx approx 126 K Phase and its Derivative Phase in the Ba-Ca-Cu-O System

摘要

Superconducting samples with T_c values as high as 126 K were obtained in the Ba-Ca-Cu-O system by a high-pressure synthesis technique. Because of the instability of the as-synthesized phase in air, it could not be identified directly. However, the structure of its derivative phase which exhibited T_c approx approx 90 K was successfully studied by electron diffraction, and found to possess body-centered (I) symmetry. The chemical composition as estimated by EDX was Ba : Ca : Cu = 2.0 : 2.1 : 3.0. TG-MS analyses carried out for both phases revealed that H_2O molecules, which were not seen in the MS profile of the 126 K-phase sample below 500 deg C, were evolved from the 90 K-phase sample around 260 deg C. Taking these results into account, we suggest that the 126 K phase has the 0223 structure, while for the 90 K phase either an H-5223:I or (H,C)-5223 structure was concluded. Refinement of synchrotron x-ray powder diffraction data for the 90 K phase gave some convergence for both H-5223:I and (H,C)-5223 structures. Irreversibility-field lines were measured for both the 126 K-phase and 90 K-phase samples.