Stable Calcium Nitrides at Ambient and High Pressures

The knowledge of stoichiometries of alkaline-earth metal nitrides, where nitrogen can exist in polynitrogen forms, is of significant interest for understanding nitrogen bonding and its applications in energy storage. For calcium nitrides, there were three known crystalline forms, CaN₂, Ca₂N, and Ca₃N₂, at ambient conditions. In the present study, we demonstrated that there are more stable forms of calcium nitrides than what is already known to exist at ambient and high pressures. Using a global structure searching method, we theoretically explored the phase diagram of CaN_x and discovered a series of new compounds in this family. In particular, we found a new CaN phase that is thermodynamically stable at ambient conditions, which may be synthesized using CaN₂ and Ca₂N. Four other stoichiometries, namely, Ca₂N₃, CaN₃, CaN₄, and CaN₅, were shown to be stable under high pressure. The predicted CaN_x compounds contain a rich variety of polynitrogen forms ranging from small molecules (N₂, N₄, N₅, and N₆) to extended chains (N_∞). Because of the large energy difference between the single and triple nitrogen bonds, dissociation of the CaN_x crystals with polynitrogens is expected to be highly exothermic, making them as potential high-energy-density materials.