Structures of the superconducting oxides Tl₂Ba₂CuO₆ and Bi₂Sr₂CuO₆

Abstract

The structures of Tl₂Ba₂CuO₆ and Bi₂Sr₂CuO₆ have been solved and refined from single-crystal x-ray diffraction data. The structures are essentially the same and have single Cu-O sheets separated by either Tl-O or Bi-O double layers. The Tl₂Ba₂CuO₆ structure is tetragonal with a=3.87 Å and c=23.24 Å, and there are strictly flat Cu-O sheets. Bi₂Sr₂CuO₆ has a lower-symmetry structure which may be approximated with an orthorhombic cell with a=5.36 Å, b=5.37 Å, and c=24.62 Å; however, this ignores superstructure reflections along both the a and c axes. The Tl-O layers are much more strongly bound to each other than are the Bi-O layers; thus, better conduction along the c axis is expected for Tl₂Ba₂CuO₆ relative to Bi₂Sr₂CuO₆. Superconducting transition temperatures of 9 and 90 K were observed for Bi₂Sr₂CuO₆ and Tl₂Ba₂CuO₆, respectively.