Crystallographic description of phases in the Y-Ba-Cu-O superconductor.

Abstract

Single-crystal x-ray techniques were used to determine two structures that comprise the Y-Ba-Cu-O superconductor. A green phase has the orthorhombic ${\mathrm{Y}}_{2}$${\mathrm{BaCuO}}_{5}$ structure. An opaque phase (${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$, 0\ensuremath{\le}x\ensuremath{\le}1.0) is tetragonal (a=3.859 A\r{}, c=11.71 A\r{}; space group P4\ifmmode\bar\else\textasciimacron\fi{}m2;Z=1). The structure is related to a 1:1:3 AB${\mathrm{O}}_{3}$ perovskite: Ba and Y order in A sites and Cu is in B sites. Systematic oxygen vacancies between adjacent copper atoms lead to a fivefold-coordinate Cu in proximity to Y and a twofold-coordinate Cu in the vicinity of the Ba site.