Stability, structural, elastic and electronic properties of polymorphs   of the superconducting disilicide YIr2Si2
  Using the ab initio approaches, the comparative stability, structural, elastic, and electronic properties of three polymorphs of the superconducting disilicide YIr2Si2, which differ in the atomic configurations of [Ir2Si2] (or [Si2Ir2]) blocks, were examined. For these YIr2Si2 polymorphs, the optimized structural data, elastic parameters, electronic bands, total and partial densities of states, Fermi surface topology, and chemical bonding have been obtained and analyzed. Our studies showed that although ThCr2Si2- and CaBe2Ge2-type polymorphs are mechanically stable and relatively hard materials with low compressibility, they will behave as ductile systems. Among them, ThCr2Si2-type polymorph will show enhanced elastic anisotropy. In the vicinity of the Fermi energy, the topology of the electronic bands and the Fermi surface for various polymorphs are quite different. Besides, the CaBe2Ge2-type polymorph is expected to be anisotropic, i.e. happening mainly in the [Si2Ir2] blocks. The inter-atomic bonding for YIr2Si2 polymorph phases can be described as an anisotropic mixture of covalent, metallic, and ionic contributions, where inside the [Ir2Si2] (or [Si2Ir2]) blocks, Ir-Si and Ir-Ir bonds take place, whereas between the adjacent [Ir2Si2] (or [Si2Ir2]) blocks and Y atomic sheets, Si-Si and Ir-Y, Si-Ir and Si-Y, or mainly Ir-Ir bonds emerge for various polymorphs. 
Igor R. Shein
cond-mat.supr-con cond-mat.mtrl-sci