Crystal chemistry of the compound Sr₂Bi₂CuO₆

The compound Sr₂Bi₂CuO₆ should nominally be the phase with n = 1 of the high T_c superconducting series Sr₂Bi₂Ca_nO_4+2n. However, the superconducting phase with n = 1 (with no CaO) occurs only with a gross deficiency in SrO content. Instead, at the composition Sr₂Bi₂CuO₆, a different phase is formed with an x-ray diffraction pattern considerably different from that expected for the n −1 member of the series. This phase has been found, by a combination of electron diffraction and single crystal and powder x-ray diffraction, to have a commensurate lattice with monoclinic symmetry, space group C2/m or Cm, a = 24.473 (2), b = 5.4223 (5), c = 21.959 (2)A, and β = 105.40 (1)°. The actual composition of this phase may be deficient in CuO by as much as 1.0 mole %.