High-T(c) superconducting phases in the series Bi2.1(Ca,Sr)(n+1)Cu(n)O(2n+4+delta)

Three distinct phases in the homologous series Bi2.1(CaSr)(n+1)Cu(n)O(2n+4+delta) with n = 1, 2, and 3 have been identified. These have zero resistances of 80 K, 91 K, and 105 K respectively. The structures in the homologous series appear to be based on alternating double bismuth Bi2O2 layers perovskite (Sr, Ca)O-CuO2 layers, with higher members obtained by intercalating additional 3.1 A CaCuO2 bilayers. The structures can be indexed on a pseudotetragonal subcell with a = b = 5.4 A and c = 24.4 A (n = 1), 30.76 A (n = 2), or 36 A (n = 3). In each structure, a 2x superlattice structure in the c-direction arises as a natural consequence of a 19/4 in commensurate structure in b direction, which accounts for 2.1 Bi atoms in the unit formula. The critical temperature for these phases is sharply dependent on the Sr/Ca ratio and oxygen stoichiometry, as determined by heat treatment.