Phase transition, structural and thermodynamic properties of Mg₂Si 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 intermediate pressure (Pnma) and high pressure phases (P6₃/mmc). The lattice constants, the band structures. The bulk moduli of the Mg₂Si polymorphs are presented and discussed. The phase transition from anti-cotunnite to Ni₂In-type Mg₂Si is successfully reproduced using a vibrational Debye-like model. The phase boundary can be described as P=24.02994+3.93×10^-3T-4.66816×10^-5T²-2.2501×10^-9T³+2.33786×10¹¹T⁴. To complete the fundamental characteristics of these polymorphs we have analysed thermodynamic properties, such as thermal expansion and heat capacity, in a pressure range of 0--40 GPa and a temperature range of 0--1300 K. The obtained results tend to support the available experimental data and other theoretical results. Therefore, the present results indicate that the combination of first principles and a vibrational Debye-like model is an efficient scheme to simulate the high temperature behaviours of Mg₂Si.