The crystal structure of the high-$T_c$ superconducting phase ${\mathrm{Ba}}_2$${\mathrm{YCu}}_3$${\mathrm{O}}_{9\mathrm{-}\mathrm{\delta }}$(δ=2.0) has been determined by single-crystal x-ray diffraction. The compound crystallizes in an orthorhombic, oxygen-deficient perovskite superstructure with space group Pmmm, a=3.856(2) Å b=3.870(2) Å, and c=11.666(3) Å. The ordering of Ba and Y in a [Ba-Y-Ba${]}_{\mathrm{\infty }}$ sequence along the c axis is responsible for the tripled cell. Two [${\mathrm{CuO}}_2$${]}_{\mathrm{\infty }}$ planes with copper in distorted square planar coordination are the key to conductivity.

• Received 16 March 1987

DOI:https://doi.org/10.1103/PhysRevB.35.7137