High-T_c superconducting phases in the series Bi_2.1(Ca, Sr)_n+lCu_nO_2n+4+δ

Abstract

Following the report of Maeda et al.¹ on high-T_c superconductivity in samples of nominal composition BiSrCaCu₂O_x we have identified three distinct phases in the homologous series Bi_2.1(Ca, Sr)_n+lCu_nO_2n+4+δ with n = 1,2 and 3. These have zero resistance T_cs 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 and perovskite (Sr, Ca)O–CuO₂ layers, with higher members obtained by intercalating additional 3.1 Å Ca-CuO₂ bilayers. The structures can be indexed on a pseudo-tetragonal subcell with a = b = 5.4 Å and c = 24.4 Å (n = 1), 30.76 Å (n = 2) or 36 Å (n = 3). We were unable to prepare the n =4 member. In each structure a 2× superlattice structure in the c-direction arises as a natural consequence of a 19/4 incommensurate structure in the b direction, which accounts for 2.1 Bi atoms in the unit formula. The critical temperature, T_c, for these phases is sharply dependent on the Sr/Ca ratio and oxygen stoichiometry, as determined by heat treatment.