Theoretical Investigation of the High-Pressure Structure, Phase Transition, and Mechanical and Electronic Properties of Mg3N2

The potential structures of magnesium nitride with a chemical composition of Mg3N2 are examined by utilizing a widely adopted evolutionary methodology for crystal structure prediction. In addition to the previously proposed phases (alpha-, beta- and gamma-Mg3N2), we find five high-pressure phases for Mg3N2: (1) a Cmc21 symmetric structure (epsilon-Mg3N2) at 27 GPa, (2) a R3 (c) over bar symmetric structure (tau-Mg3N2) at 30 GPa, (3) a Cmcm symmetric structure (omega-Mg3N2) at 53 GPa, (4) a Ima2 symmetric structure (lambda-Mg3N2) at 68 GPa, and (5) a Ibam symmetric structure (mu-Mg3N2) at 115 GPa. All these phases are mechanically and dynamically stable by checking the elastic constants and phonon dispersion. All phases are direct band gap semiconductors except that the omega-Mg3N2 phase has a nontrivial indirect band gap. Mechanical properties calculations reveal that the gamma-Mg3N2 phase has superior ductility than other phases. The Vickers hardness of each phase has been evaluated to be about 15 GPa based on an empirical relation.