Periodic DFT calculation of the pressure-induced phase transition and thermodynamical properties of magnesium silicide polymorphs

The plane-wave pseudo-potential method within the framework of first-principles is used to investigate the structural and elastic properties of Mg2Si in its low pressure phase (Fm-3m) and intermediate pressure phase (Pnma). The high-pressure lattice constants, the elastic constants, the elastic moduli and the anisotropy factors of the anti-cotunnite Mg2Si are presented and discussed. The results show that our system is mechanically stable. The reversible phase transition from anti-fluorite to anti-cotunnite structure is successfully reproduced through the quasi-harmonic Debye model. The phase boundary can be described as P=4.06826−6.95×10−3T+5.08838×10−5T2−4.24073×10−8T3. To complete the fundamental characteristics of these compounds we have analysed the thermodynamic properties such as thermal expansion, bulk modulus, isochoric heat capacity and Debye temperature in a pressure range 0–21 GPa and a temperature range 0–1200 K. The obtained results tend to support the experimental data when available. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic approximations is an efficient scheme to simulate the high-temperature behaviours of semiconductors like Mg2Si.