We report superconductivity in defective pyrite-type iridium chalcogenides ${\mathrm{Ir}}_x{\mathrm{Ch}}_2$ ($\mathrm{Ch}=\mathrm{Se}$ and Te). Maximum values of $T_c$ of 6.4 K for ${\mathrm{Ir}}_{0.91}{\mathrm{Se}}_2$ and 4.7 K for ${\mathrm{Ir}}_{0.93}{\mathrm{Te}}_2$ were observed. It was found that ${\mathrm{Ir}}_{0.75}{\mathrm{Ch}}_2$ (${\mathrm{Ir}}_3{\mathrm{Ch}}_8$) is close to the boundary between metallic and insulating states and ${\mathrm{Ir}}_x{\mathrm{Ch}}_2$ 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 ${\mathrm{Ir}}_{0.75}{\mathrm{Ch}}_2$ (${\mathrm{Ir}}_3{\mathrm{Ch}}_8$) is the parent compound for the present superconductors.

• Received 5 August 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.217002