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 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 highpressure 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).