Pressure-induced stability and polymeric nitrogen in alkaline earth metal N-rich nitrides (XN 6 , X = Ca, Sr and Ba): a first-principles study.

Multi-nitrogen or polynitrogen compounds can be used as potential high energy-density materials, so they have attracted great attention. Nitrogen can exist in alkaline earth metal nitrogen-rich (N-rich) compounds in the form of single or double bonds. In recent years, to explore N-rich compounds which are stable and easy to synthesize has become a new research direction. The N-rich compounds XN 6 (X = Ca, Sr and Ba) have been reported under normal pressure. In order to find other stable crystal structures, we have performed XN 6 (X = Ca, Sr and Ba) exploration under high pressure. We found that SrN 6 has a new P 1̄ phase at a pressure of 22 GPa and an infinite nitrogen chain structure, and BaN 6 has a new C 2/ m phase at 110 GPa, with an N 6 ring network structure. Further, we observed that the infinite nitrogen chain and the N 6 ring network structure contain typical covalent bonds formed by the hybridization of the sp 2 and sp 3 orbitals of N, respectively. It is found that both SrN 6 and BaN 6 are semiconductor materials and the N-2p orbital plays an important role in the stability of the crystal structure for P 1̄-SrN 6 and C 2/ m -BaN 6 . Because of the polymerization of nitrogen in the two compounds and their stabilities under high pressure, they can be used as potential high energy-density materials. The research in this paper further promotes the understanding of alkaline earth metal N-rich compounds and provides new information and methods for the synthesis of alkaline earth metal N-rich compounds (XN 6 , X = Ca, Sr and Ba).