Electronic structure and chemical bonding of α- and β-CeIr2Si2 intermediate valence compounds

The dimorphism of the intermediate valence ternary cerium silicide CeIr2Si2 in the ThCr2Si2 (α) and CaBe2Ge2 (β) modifications is addressed in the framework of the density functional theory. The geometry optimization is in good agreement with the experiment and the subsequent establishment of the energy–volume equation of state (EOS) indicates a stabilization of the β-type relative to the α-type concomitant with the trend of the cerium valence, changing to tetravalent in β-CeIr2Si2. This is equally shown from the site projected DOS and from the large increase of the electronic contribution to the specific heat. The chemical bonding indicates the strongest bonding interactions within the Ir–Si substructure in both varieties. Stabilization of β-CeIr2Si2 with almost tetravalent cerium is in good agreement with ThIVIr2Si2 which exclusively crystallizes in the CaBe2Ge2 type. The EOS behavior of different RIr2Si2 (R=Th, Ce, La) is comparatively discussed.