Oxygen and cation nonstoichiometry in Tl-based superconductors
Abstract Oxygen nonstoichiometry and cation substitution in Tl-based superconductors were investigated. It was demonstrated that Tc variations observed in these compounds were attributed to oxygen nonstoichiometry and the corresponding change in hole concentration. Tl2Ba2CuO6 and Tl2Ba2CaCu2O8 are over-doped with hole carriers, and the decrease in oxygen content brings about an appropriate carrier concentration for high-Tc superconductivity. In particular, the Tl2Ba2CuO6 compound has excess oxygen atoms at an interstitial site between double TlO layers, and its reversible occupation controls hole carriers at the CuO2 layer. Substitution of Cu for Tl-site in Tl2Ba2CuO6 was also suggested. A significant portion of hole carriers in this compound can be explained by incorporation of excess oxygen and substitution of Cu for Tl.