Phase transition and properties of ternary MgGeN 2 under pressure: a first principles investigation

In this work the structural, electronic, mechanical and optical properties of MgGeN 2 under pressure are investigated through the density functional theory based first principles computations using the recently proposed Strongly Constrained and Appropriately Normed (SCAN) functional. It was found that the orthorhombic structure is energetically stable at ambient conditions and a phase transition from orthorhombic phase to a newly found tetragonal phase occurs at about 23 GPa under hydrostatic compression. In addition, in the investigated pressure range, MgGeN 2 does not follow the rule that the materials will become more metallic under pressure due to that the strengthened covalent bond will enlarge the band gap and enable the system more insulating. Therefore, for both the orthorhombic phase and the high pressure tetragonal phase, the band gap shows a monotonic increment along increasing pressure. A reduction of the band gap was accompanied with the phase transition. In addition, the high pressure phase has a much larger bulk modulus and shear modulus than the orthorhombic phase, together with an enhanced optical absorption and conductivity. Finally, the potential applications of pressure induced structural change and band tuning are interpreted.