Superconductivity in defective pyrite-type iridium chalcogenides Ir(x)Ch2 Ch = Se and Te.

We report superconductivity in defective pyrite-type iridium chalcogenides Ir(x)Ch2 (Ch = Se and Te). Maximum values of T(c) of 6.4 K for Ir(0.91)Se(2) and 4.7 K for Ir(0.93)Te(2) were observed. It was found that Ir(0.75)Ch(2) (Ir(3)Ch(8)) is close to the boundary between metallic and insulating states and Ir(x)Ch(2) systems undergo nonmetal to metal transitions as x increases. On the basis of density functional theory calculations and the observed large variation in the Ch-Ch distance with x, we suggest that Ir(0.75)Ch(2) (Ir(3)Ch(8)) is the parent compound for the present superconductors.