Abstract
Oxygen nonstoichiometry and cation substitution in Tl-based superconductors were investigated. It was demonstrated that T c variations observed in these compounds were attributed to oxygen nonstoichiometry and the corresponding change in hole concentration. Tl 2Ba 2CuO 6 and Tl 2Ba 2CaCu 2O 8 are over-doped with hole carriers, and the decrease in oxygen content brings about an appropriate carrier concentration for high- T c superconductivity. In particular, the Tl 2Ba 2CuO 6 compound has excess oxygen atoms at an interstitial site between double TlO layers, and its reversible occupation controls hole carriers at the CuO 2 layer. Substitution of Cu for Tl-site in Tl 2Ba 2CuO 6 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.
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