  A new superconductor (Na0.25K0.45)Ba3Bi4O12, having an A-site-ordered double
perovskite structure, with a maximum Tc ~ 27 K has very recently been
discovered through hydrothermal synthesis at 593 K. The structural, elastic,
electronic, and thermal properties of the new synthesized compound have been
investigated theoretically. Here we have employed the pseudo-potential
plane-wave (PP-PW) approach based on the density functional (DFT) theory,
within the generalized gradient approximation (GGA). The elastic constants
(Cij), Pugh`s ratio, Cauchy`s pressure and other elastic parameters are derived
and analyzed using energy strain method for the first time. We have discussed
the bonding nature in the light of the electronic valence charge density. Both
electron and hole-like Fermi surfaces are present in the compound under study
which indicate the multiple-band nature of (Na0.25K0.45)Ba3Bi4O12. The compound
is indicated to be a strongly coupled superconductor which is based on the
estimated e-ph coupling constant. The thermodynamic properties such as bulk
modulus, Debye temperature, heat capacities and volume thermal expansion
coefficient at elevated temperature and pressure are calculated and analyzed
for the first time by using quasi-harmonic model.
  Key words: double perovskite, superconductor, elastic properties, electronic
properties, thermal properties
