  We have carried out a first principles study of the elastic properties and
electronic structure for two room-temperature stable Pt silicide phases,
tetragonal alpha-Pt_2Si and orthorhombic PtSi. We have calculated all of the
equilibrium structural parameters for both phases: the a and c lattice
constants for alpha-Pt_2Si and the a, b, and c lattice constants and four
internal structural parameters for PtSi. These results agree closely with
experimental data. We have also calculated the zero-pressure elastic constants,
confirming prior results for pure Pt and Si and predicting values for the six
(nine) independent, non-zero elastic constants of alpha-Pt_2Si (PtSi). These
calculations include a full treatment of all relevant internal displacements
induced by the elastic strains, including an explicit determination of the
dimensionless internal displacement parameters for the three strains in
alpha-Pt_2Si for which they are non-zero. We have analyzed the trends in the
calculated elastic constants, both within a given material as well as between
the two silicides and the pure Pt and Si phases. The calculated electronic
structure confirms that the two silicides are poor metals with a low density of
states at the Fermi level, and consequently we expect that the Drude component
of the optical absorption will be much smaller than in good metals such as pure
Pt. This observation, combined with the topology found in the first principles
spin-orbit split band structure, suggests that it may be important to include
the interband contribution to the optical absorption, even in the infrared
region.
