Crystal structure of the high-Tc superconductor Ba2YCu3O9- delta.

Abstract

The crystal structure of the high-${T}_{c}$ superconducting phase ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{9\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$(\ensuremath{\delta}=2.0) has been determined by single-crystal x-ray diffraction. The compound crystallizes in an orthorhombic, oxygen-deficient perovskite superstructure with space group Pmmm, a=3.856(2) A\r{} b=3.870(2) A\r{}, and c=11.666(3) A\r{}. The ordering of Ba and Y in a [Ba-Y-Ba${]}_{\mathrm{\ensuremath{\infty}}}$ sequence along the c axis is responsible for the tripled cell. Two [${\mathrm{CuO}}_{2}$${]}_{\mathrm{\ensuremath{\infty}}}$ planes with copper in distorted square planar coordination are the key to conductivity.