  Very recently (November, 2010, PRB, 82, 180520R) the first 122-like ternary
superconductor KxFe2Se2 with enhanced TC ~ 31K has been discovered. This
finding has stimulated much activity in search of related materials and
triggered the intense studies of their properties. Indeed already in 2010-2011
the superconductivity (TC ~ 27-33K) was also found in the series of new
synthesized 122 phases such as CsxFe2Se2, RbxFe2Se2, (TlK)xFeySe2 etc. which
have formed today the new family of superconducting iron-based materials
without toxic As. Here, using the ab initio FLAPW-GGA method we have predicted
for the first time the elastic properties for KFe2Se2 and discussed their
interplay with inter-atomic bonding for this system. Our data reveal that the
examined phase is relatively soft material. In addition, this system is
mechanically stable, adopts considerable elastic anisotropy, and demonstrates
brittleness. These conclusions agree with the bonding picture for KFe2Se2,
where the inter-atomic bonding is highly anisotropic and includes ionic,
covalent and metallic contributions.
