Aurivillius-Popper mixed superconductors in BiO-CuO-(Sr_0.5, Ca_0.5)O system

We report the effect of inhomogeneities on the electrical resistivity and ac magnetic susceptibility in Aurivillius-like bismuth mixed phase oxides of the BiO-CuO-(Sr_0.5, Ca_0.5)O system and propose a crystal structure of the major phase having highest T_c. Nominal Aurivillius compositions with molar ratios of BiO/(Sr_0.5, Ca_0.5)O = 1/2 are superconductors with T_cranging from 83 to 107 K, and are accompanied by a large expansion during sintering due to the formation of Kirkendall voids. T_cincreases with decreasing of the c lattice parameter. An oxide BiSrCaCu₂O_x(n = 2) shows a maximum T_cvalue of 107 K and an onset of superconductivity at a much higher temperature. It seems that the structure of Bi₂Sr₂CaCu₂O_xconsists of an Aurivillius-like phase having two perovskite layers and a Popper mixed phase. The ac magnetic susceptibility showed an overall decrease in susceptibility with time up to 220 days. This appears to be related to the relief of intralattice strain.