Structures of the superconducting oxides ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$Cu${\mathrm{O}}_6$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$Cu${\mathrm{O}}_6$

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$${\mathrm{Ba}}_2$Cu${\mathrm{O}}_6$ structure is tetragonal with $a=3.87\mathrm{}$ Å and $c=23.24\mathrm{}$ Å, and there are strictly flat Cu-O sheets. ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$Cu${\mathrm{O}}_6$ has a lower-symmetry structure which may be approximated with an orthorhombic cell with $a=5.36\mathrm{}$ Å, $b=5.37\mathrm{}$ Å, and $c=24.62\mathrm{}$ 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 ${\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.