Superconductivity in the YIr2Si2 and LaIr2Si2 Polymorphs

We report on the existence of superconductivity in YIr2Si2 and LaIr2Si2 compounds in relation to the crystal structure. The two compounds crystallize in two structural polymorphs, both tetragonal. The high-temperature polymorph (HTP) is of the CaBe2Ge2 structure type (space group P4/nmm), while the low-temperature polymorph (LTP) is of the ThCr2Si2 structure type (I4/mmm). By studying the polycrystals prepared by arc melting, we observed that the rapidly cooled samples retain the HTP even at room temperature (RT) and below. Annealing such samples at >= 900 degrees C followed by slow cooling to RT provides the LTP. Both the HTP and LTP were subsequently studied with respect to magnetism and superconductivity by electrical resistivity, magnetization, AC susceptibility, and specific heat measurements. The HTP and LTP of both compounds behave as Pauli paramagnets. Superconductivity was found exclusively in the HTP of both compounds below T-SC (= 2.52 K in YIr2Si2 and 1.24 K in LaIr2Si2). The relationships of magnetism and superconductivity with the electronic and crystal structures are discussed by comparing experimental data with the results of first-principles electronic structure calculations.