A new family of superconducting silicides: The ThMxSi2−x compounds (M = Rh or Ir with 0 < x < 1) of α-ThSi2-type structure

New superconducting materials have been prepared by substitution of silicon by Rh or Ir atoms in the α-ThSi₂ disilicide. The ThM_xSi_2?x compounds crystallize in the tetragonal α-ThSi₂-type structure with a large homogeneity range: 0? x ? 0.96 for Rh and 0? x ? 1 for Ir. A sharp increase of the superconducting transition temperature T_cr is observed for x > 0.75 in both cases. Among the superconducting compounds with an α-ThSi₂-type structure, ThRh_0.96Si_1.04 and ThIrSi have the highest T_cr values with respectively 6.45 and 6.5 K about twice that of α-ThSi₂ (T_cr = 3.16 K). Superconductivity enhancement has been attributed to the formation of Rh-Rh or Ir-Ir metallic type bonding.