Stable Calcium Nitrides at Ambient and High Pressures

Abstract

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 CaNx 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 CaNx 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 CaNx crystals with polynitrogens is expected to be highly exothermic, making them as potential high-energy-density materials.

Formation mechanism of insensitive tellurium hexanitride with armchair-like cyclo-N6 anions

“…However, the cyclo-N 6 ionic salt only remains in the theoretical stage. Numerous planar or quasiplanar cyclo-N 6 anions are predicted at high pressure in Li, Mg, Cs, Ca, Rb, and Ba nitrides, but they have not been successfully synthesized for unclear reasons 6,[22][23][24][25] .…”

High‐Pressure Synthesis of a Nitrogen‐Rich Inclusion Compound ReN8⋅x N2 with Conjugated Polymeric Nitrogen Chains

“…Chains of as imilar type were recently predicted in CaN 4 [33] (h 4 -coordination together with metallocyclic coordination), MgN 4 , [18,34] HfN 10 , [35] and in some hydrogen-nitrogen compounds. Chains of as imilar type were recently predicted in CaN 4 [33] (h 4 -coordination together with metallocyclic coordination), MgN 4 , [18,34] HfN 10 , [35] and in some hydrogen-nitrogen compounds.…”

“…To the best of our knowledge,R eN 8 ·x N 2 is the first synthetic compound that contains polydiazenediyl chains. Chains of as imilar type were recently predicted in CaN 4 [33] (h 4 -coordination together with metallocyclic coordination), MgN 4 , [18,34] HfN 10 , [35] and in some hydrogen-nitrogen compounds. [36] Interestingly,the structure of HfN 10 as predicted [35] is similar to the crystal structure of ReN 8 ·x N 2 .R ecently,w e reported an ovel compound FeN 4 containing polymeric nitrogen chains of ad ifferent type.T hese chains consist of sp 2 -a nd sp 3 -hybridized nitrogen atoms forming catena-poly-[tetraz-1-ene-1,4-diyl] chains.…”

High‐Pressure Synthesis of Metal–Inorganic Frameworks Hf4N20⋅N2, WN8⋅N2, and Os5N28⋅3 N2 with Polymeric Nitrogen Linkers

“…Recently synthesized MgN 4 featuring polydiazenediyl chains could be even recovered at ambient conditions . Polydiazenediyl chains are also theoretically predicted to exist in CaN 4 , ReN 4 , and HfN 8 ⋅N 2 . Along with a pentazolate anion, the polydiazenediyl chain may be one of the most stable structural units to be preserved at ambient pressures.…”