Bulk synthesis of the 81-K superconductor YBa₂Cu₄O₈ at high oxygen pressure

Abstract

The superconducting phase YBa₂Cu₄O₈ ('124') differs from YBa₂Cu₃O₇ ('123') in having a double, instead of single, Cu–O chain running parallel to the b axis. It was first observed as a lattice defect in partly decomposed 123 powders¹, and then as an ordered defect structure in inhomogeneous 123 thin films^2,3, but has not up to now been synthesized in bulk. As part of a systematic study of the pressure–temperature–composition phase diagram of pseudo-binary Y–Ba–Cu–O systems at high oxygen pressure^4,5, we have determined the approximate field of thermodynamic stability of the 124 phase. Here we report synthesis of the phase in bulk at 400 bar O₂ and 1,040 ^°C. In agreement with previous measurements in thin films, the transition temperature is 81 K. Unlike the 123 compound, the oxygen content in 124 is thermally stable up to 850 ^°C, which may prove important for applications.