Energy density of high-pressure nitrogen-rich MN_x compounds

In the past decade, a large number of nitrogen-rich MN_x compounds have been discovered under high-pressure conditions. In this work, we have evaluated the energy densities of MN_x structures with thermodynamic and dynamical stability through first-principles calculations. The results show that the energy densities of MN_x consisting of alkali metals and cyclo-N₅^- are less than ∼0.5 TNT equivalence, whereas the group-III metal nitrides have high-energy density regardless of the type of nitrogen oligomers in the structures. To clarify the energy density difference for MN_x composed of different impurities, bivariate Pearson correlation analysis is performed, which reveals that the high-energy density of MN_x is related to the large N density, small M atomic radius, short M-N bond length, small MN_x ionicity, long N-N bond length and large M formal oxidation state. According to this correlation, H, Be, B, Al, Si and P elements have been proposed as the candidate impurities to synthesize high-energy density MN_x.