Emergence of Novel Polynitrogen Molecule-like Species, Covalent Chains, and Layers in Magnesium–Nitrogen MgxNy 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 N64– 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-N42–, 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).