Superconductivity in a new ternary structure class of boride compounds

The occurrence of superconductivity at temperatures up to 9.6 K is reported for the system M(Rh_0.85Ru_0.15)₄B₄, where M can be Y or a rare earth element. In contrast, most members of the isostructural series MRu₄B₄ are either not superconducting or order magnetically instead. These materials were found to crystallize in a body-centered-tetragonal structure, with space group I⁴₁/acd and 8 formula units per unit cell. The atomic positions were determined through a structural study of the LuRu₄B₄ compound. These results are discussed with reference to the previously reported primitive tetragonal MRh₄B₄ system of superconducting or ferromagnetic compounds.