Aurivillius-Popper mixed superconductors in BiO–CuO–(Sr0.5, Ca0.5)O system

Abstract

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–(Sr0.5,Ca0.5)O system and propose a crystal structure of the major phase having highest Tc. Nominal Aurivillius compositions with molar ratios of BiO/(Sr0.5,Ca0.5)O = 1/2 are superconductors with Tc ranging from 83 to 107 K, and are accompanied by a large expansion during sintering due to the formation of Kirkendall voids. Tc increases with decreasing of the c lattice parameter. An oxide BiSrCaCu2Ox (n = 2) shows a maximum Tc value of 107 K and an onset of superconductivity at a much higher temperature. It seems that the structure of Bi2Sr2CaCu2Ox consists 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.