Synthesis and structure of high temperature superconducting powders

The chemistry of the synthesis of high temperature superconductors is particularly challenging. Some of the very difficult materials problems center around the fact that all of the evidence to date indicates that the high temperature superconductors are thermo-dynamically unstable compounds. It is interesting to note that there is a fully oxygenated nonsuperconducting precursor phase (T') in the Y-Ba-Cu-O system. This important T' phase has been largely overlooked in the synthesis and subsequent properties of the superconducting orthorhombic YBaCu3O7-x. The T' nonsuperconducting phase is tetragonal and is a fully oxygenated polymorph of the superconducting orthorhombic YBa2Cu3O7-x phase. This tetragonal polymorph of YBa2Cu3O7-x has a larger a-axis and a smaller c-axis than the tetragonal YBa2Cu3O6 phase. (author). 14 refs.; 4 figs.; 2 tabs