A study of the phase transitions, electronic structures and optical properties of Mg2Si under high pressure

The phase transitions, electronic structures and optical properties of Mg2Si under high pressure were researched by using the first-principles density functional theory. The calculated results demonstrate that Mg2Si undergoes two pressure-induced phase transitions: one is the transition from the cubic anti-fluorite (Fm3̄m) structure to the orthorhombic anti-cotunnite (Pnma) structure at 8.38 GPa, and the other is the transition from the orthorhombic anti-cotunnite structure to the hexagonal Ni2In-type (P63m̄mc) structure at 28.84 GPa. A good agreement between the calculated transition pressures and the previous experimentally measured results has been found. The structural parameters of the high-pressure phases are characterized in detail; the discrepancy between the calculated and experimental results for the anti-cotunnite phase is pointed out and analyzed. Finally, the electronic and optical properties of both the ambient anti-fluorite and the high-pressure phases are discussed; they show that Mg2Si has become metallic at high pressure.