Elastic, vibrational and thermodynamic properties of α-Snα-Sn based group IV semiconductors and GeC under pressure

Author-Highlights
• Elastic, vibrational and thermodynamic properties are calculated. C44 softens as pressure increases for SnSi and GeSn.
• Pressure and temperature dependences of bulk modulus and thermal expansion are reported.
• Thermal expansion coefficient shows negative behavior for SnSi and SnGe.
• The thermal expansion coefficient increases then decreases with tin concentration in SnxGe1−x alloy.

We present first-principles calculations of the structural, elastic, vibrational and thermodynamic properties of SnSi, SnGe, SnC and GeC. We employ the density-functional perturbation theory (DFPT) within the local density approximation in conjunction with the quasi-harmonic approximation. The calculated lattice parameters, which are obtained by minimizing the total energy, are in the range of those reported in the literature for the binary compounds and in good agreement with the measured ones for the elemental components. Our results for the elastic properties show that c44 softens as pressure increases for SnSi and SnGe. The phonon spectra, the density of states and the Born effective charge at zero pressure are calculated and the phonon frequencies are positive. A pressure induced soft transverse acoustic phonon mode is identified at the zone boundary X point of the Brillouin zone at pressure of 12.95 and 12.45 GPa for SnSi and SnGe respectively. The linear expansion coefficient for the elemental components is calculated and compared to experiment. The temperature and pressure dependence of the thermal expansion, the overall Grüneisen parameter, the bulk modulus and the heat capacity is predicted. The thermal expansion coefficient decreases with increasing pressure and does not show any negative behavior for GeC and this is due to the positive transverse acoustic mode Grüneisen parameters. Our results for SnxGe1−xSnxGe1−x alloys using the supercell method indicate that the variation of the Grüneisen parameter and the thermal expansion with concentration has the same trend and the bulk modulus softens.