Emergence of Novel Polynitrogen Molecule-like Species, Covalent Chains, and Layers in Magnesium-Nitrogen Mg_xN_y Phases under High Pressure

Stable structures and stoichiometries of binary Mg-N compounds are explored at pressures from ambient up to 300 GPa using ab initio evolutionary simulations. In addition to Mg₃N₂, we identified five nitrogen-rich compositions (MgN₄, MgN₃, MgN₂, Mg₂N₃, and Mg₅N₇) and three magnesium-rich ones (Mg₅N₃, Mg₄N₃ and Mg₅N₄), which have stability fields on the phase diagram. These compounds have peculiar structural features, such as N₂ dumbbells, bent N₃ units, planar SO₃-like N(N)₃ units, N₆ six-membered rings, 1D polythiazyl S₂N₂-like nitrogen chains, and 2D polymeric nitrogen nets. The dimensionality of the nitrogen network decreases as magnesium content increases; magnesium atoms act as a scissor by transferring valence electrons to the antibonding states of nitrogen sublattice. In this context, pressure acts as a bonding glue in the nitrogen sublattice, enabling the emergence of polynitrogen molecule-like species and nets. In general, Zintl-Klemm concept and molecular orbital analysis proved useful for rationalizing the structural, bonding and electronic properties encountered in the covalent nitrogen-based units. Interestingly, covalent six-membered N₆^4- rings containing P-1 (I) MgN₃ phase is recoverable at atmospheric pressure. Moreover, ab initio molecular dynamics analysis reveals the polymeric covalent nitrogen network, poly-N₄^2-, encountered in the high-pressure Cmmm MgN₄ phase can be preserved at ambient conditions. Thus, quenchable MgN₄, stable at pressures above 13 GPa, shows that high energy-density materials based on polymeric nitrogen can be achievable at reduced pressures. The high-pressure phase P-1 (I) MgN₃ with covalent N₆ rings is the most promising HEDM candidate with an energy density of 2.87 kJ·g^-1, followed by P-1 MgN₄ (2.08 kJ·g^-1).