Emergence of Novel Polynitrogen Molecule-like Species, Covalent Chains, and Layers in Magnesium–Nitrogen Mg<sub><i>x</i></sub>N<sub><i>y</i></sub> 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 Mg<sub>3</sub>N<sub>2</sub>, we identified five nitrogen-rich compositions (MgN<sub>4</sub>, MgN<sub>3</sub>, MgN<sub>2</sub>, Mg<sub>2</sub>N<sub>3</sub>, and Mg<sub>5</sub>N<sub>7</sub>) and three magnesium-rich ones (Mg<sub>5</sub>N<sub>3</sub>, Mg<sub>4</sub>N<sub>3</sub> and Mg<sub>5</sub>N<sub>4</sub>), which have stability fields on the phase diagram. These compounds have peculiar structural features, such as N<sub>2</sub> dumbbells, bent N<sub>3</sub> units, planar SO<sub>3</sub>-like N­(N)<sub>3</sub> units, N<sub>6</sub> six-membered rings, 1D polythiazyl S<sub>2</sub>N<sub>2</sub>-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 N<sub>6</sub><sup>4–</sup> rings containing <i>P</i>–1 (I) MgN<sub>3</sub> phase is recoverable at atmospheric pressure. Moreover, ab initio molecular dynamics analysis reveals the polymeric covalent nitrogen network, poly-N<sub>4</sub><sup>2–</sup>, encountered in the high-pressure <i>Cmmm</i> MgN<sub>4</sub> phase can be preserved at ambient conditions. Thus, quenchable MgN<sub>4</sub>, 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 <i>P</i>–1 (I) MgN<sub>3</sub> with covalent N<sub>6</sub> rings is the most promising HEDM candidate with an energy density of 2.87 kJ·g<sup>–1</sup>, followed by <i>P</i>–1 MgN<sub>4</sub> (2.08 kJ·g<sup>–1</sup>).