Mg$_2$Si silicide under pressure: first-principles evolution search results

The search for optimal Mg$_2$Si silicide structures has been performed using the software suite implementing the evolution algorithms on the basis of density functional theory (DFT). It has been shown that the well-known hexagonal structure of $P$6$_3/mmc$ symmetry is converted under the pressure $P\sim$ 34 GPa into a monoclinic structure of $C$2/$m$ symmetry, which is stable up to the pressures $P<$ 76 GPa. This structure is replaced by the orthorhombic Pmmm structure, which is retained up to the pressures $P\sim$ 235 GPa and further gives way to a monoclinic structure of $P$2/$m$ symmetry. It is demonstrated how the structural transitions $P$6$_{3}/mmc\to C$2/$m\to Pmmm\to P$2/$m$ occur under pressure at the atomic level. The structural phase diagram of Mg$_2$Si silicide was plotted within a pressure range of 0–240 GPa.