La${\mathrm{Ir}}_2$${\mathrm{Si}}_2$ has a high-temperature form crystallizing in the primitive tetragonal Ca${\mathrm{Be}}_2$${\mathrm{Ge}}_2$-type structure (space group $\frac{P4\mathrm{}}{\mathrm{nmm}}$; $a=4.191\mathrm{}$ Å, $c=9.944\mathrm{}$ Å), and a low-temperature form of the body-centered tetragonal Th${\mathrm{Cr}}_2$${\mathrm{Si}}_2$ type (space group $\frac{I4\mathrm{}}{\mathrm{mmm}}$; $a=4.109\mathrm{}$ Å, $c=10.299\mathrm{}$ Å). Both structures are Ba${\mathrm{Al}}_4$-type derivatives. The high-temperature modification may be quenched from above the transformation temperature of 1720°C and becomes superconducting at 1.6 K, while the low-temperature modification is normal down to 1 K. The structures of both modifications have been refined in single-crystal studies. The exceptional polymorphism of La${\mathrm{Ir}}_2$${\mathrm{Si}}_2$ is discussed, emphasizing structural relationships and the occurrence of superconductivity in the high-temperature modification.

• Received 28 December 1982

DOI:https://doi.org/10.1103/PhysRevB.28.1389