Impact of NitrogenPolymerization on the Propertiesof MgN₂ Polymorphs: Ultrasoft Semiconductor versus HardWide-Bandgap Semiconductor

Utilizing evolutionary algorithms and density functional theory calculations, we predict two novel semiconducting phases of MgN₂ (<i>C</i>2/<i>m</i>-MgN₂ and <i>R</i>3̅-MgN₂) that are dynamically stable at ambient pressure and thermally stable at 300 and 600 K. Interestingly, the different polymerization forms of nitrogen in the <i>C</i>2/<i>m</i> and <i>R</i>3̅ phases lead to significantly different mechanical properties. Specifically, <i>C</i>2/<i>m</i>-MgN₂ is an exceptionally rare ultrasoft semiconductor (<i>H</i>_<i>v</i> = 0.11 GPa, lower than aluminum and silver) with high anisotropy (<i>A</i>^<i>U</i> = 24.85), while <i>R</i>3̅-MgN₂ is a hard semiconductor (<i>H</i>_<i>v</i> = 17.72 GPa) with a low melting point (<i>T</i>_<i>m</i> = 1057 ± 13 K, nearly half that of <i>P</i>6₃<i>mc</i>-AlN and <i>P</i>31<i>c</i>-Si₃N₄). Further analysis of explosive performance revealed that <i>R</i>3̅-MgN₂ has a higher energy density than <i>Cmmm</i>-MgN₄, despite the latter having a higher nitrogen content, due to the unique wrinkled six-membered N₆ rings in <i>R</i>3̅-MgN₂. These findings provide valuable insights into the impact of nitrogen polymerization on the mechanical and explosive performances of nitrogen-rich metal nitrides and offer guidance for designing nitrides with tailored properties.