title: 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.
authors.affiliation: Institute of Atomic and Molecular Physics, Sichuan University , Chengdu 610065, China.
authors.forename: Shuangshuang
authors.initials: S
authors.lastname: Zhu
authors.affiliation: College of Physics and Electronic Information, Luoyang Normal University , Luoyang 471022, China.|Beijing Computational Science Research Center , Beijing 10084, China.
authors.forename: Feng
authors.initials: F
authors.lastname: Peng
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington , 5251 Broad Branch Road NW, Washington, D.C. 20015, United States.
authors.forename: Hanyu
authors.initials: H
authors.lastname: Liu
authors.affiliation: Department of Physics and Engineering Physics, University of Saskatchewan , Saskatoon, Saskatchewan S7N 5E2, Canada.
authors.forename: Arnab
authors.initials: A
authors.lastname: Majumdar
authors.affiliation: Institute of Atomic and Molecular Physics, Sichuan University , Chengdu 610065, China.
authors.forename: Tao
authors.initials: T
authors.lastname: Gao
authors.affiliation: Department of Physics and Engineering Physics, University of Saskatchewan , Saskatoon, Saskatchewan S7N 5E2, Canada.|Canadian Light Source , Saskatoon, Saskatchewan S7N 2V3, Canada.
authors.forename: Yansun
authors.initials: Y
authors.lastname: Yao