  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.
