Structural phase transformations of Mg3N2 at high pressure : experimental and theoretical studies

The structural behavior of Mg₃N₂ has been investigated up to 40.7 GPa at room temperature by means of angle-dispersive X-ray diffraction. A reversible, first-order structural phase transition from the ambient cubic phase (Ia3̅) to a high-pressure monoclinic phase (C2/m) is found to start at ~ 20.6 GPa and complete at ~ 32.5 GPa for the first time. The equation of state determined from our experiments yields bulk moduli of 110.7(2) and 171.5(1) GPa for the cubic and monoclinic phases, respectively, indicating higher incompressibility of the high-pressure phase of Mg₃N₂. First-principles calculations reproduced the phase stability and transition pressure determined in our experiment. In addition, a second phase transition from the monoclinic phase to a hexagonal phase (P3̅m₁) was predicted around 67 GPa for Mg₃N₂. The electronic band structures of three phases of Mg₃N₂ are also calculated and discussed.