Structures of the superconducting oxides Tl2Ba2CuO6 and Bi2Sr2CuO6.

Abstract

The structures of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$Cu${\mathrm{O}}_{6}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Cu${\mathrm{O}}_{6}$ 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 ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$Cu${\mathrm{O}}_{6}$ structure is tetragonal with $a=3.87$ \AA{} and $c=23.24$ \AA{}, and there are strictly flat Cu-O sheets. ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Cu${\mathrm{O}}_{6}$ has a lower-symmetry structure which may be approximated with an orthorhombic cell with $a=5.36$ \AA{}, $b=5.37$ \AA{}, and $c=24.62$ \AA{}; 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 ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$Cu${\mathrm{O}}_{6}$ relative to ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Cu${\mathrm{O}}_{6}$. Superconducting transition temperatures of 9 and 90 K were observed for ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Cu${\mathrm{O}}_{6}$ and ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$Cu${\mathrm{O}}_{6}$, respectively.