First-principles elastic constants and electronic structure of $\alpha -{\mathrm{Pt}}_2\mathrm{Si}$ and PtSi

We have carried out a first-principles study of the elastic properties and electronic structure for two room-temperature stable Pt silicide phases, tetragonal $\alpha -{\mathrm{Pt}}_2\mathrm{Si},$ and orthorhombic PtSi. We have calculated all of the equilibrium structural parameters for both phases: the a and c lattice constants for $\alpha -{\mathrm{Pt}}_2\mathrm{Si}$ 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, nonzero elastic constants of $\alpha -{\mathrm{Pt}}_2\mathrm{Si}$ (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 $\alpha -{\mathrm{Pt}}_2\mathrm{Si}$ for which they are nonzero. We have analyzed the trends in the calculated elastic constants, both within each material as well as among 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.