Crystal structure of new superconducting materials LaIrSi3 and LaRhSi3. Structural relation between LaRh2Si2, La2Rh3Si5 and LaRhSi3

New superconducting ternary silicides LaRhSi₃ and LaIrSi₃ have been prepared by arc melting from the elements and annealing for ten days at 900°C. A single crystal study shows these materials to be of tetragonal symmetry and isostructural with BaPtSn₃. Their superconducting transition temperature which depends strongly on stoichiometry and thermal treatment occurs between 1.9 K and 2.7 K. The structures have been compared to those of the superconducting compounds LaRh₂Si₂ and La₂Rh₃Si₅. The occurence of superconductivity in these materials seems to be related to the presence of similar coordination polyhedra for Rh or Ir and Si atoms.