Pressure-induced stability and polymeric nitrogen in alkaline earth metal N-rich nitrides (XN6, X = Ca, Sr and Ba): a first-principles study

Abstract

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 XN₆ (X = Ca, Sr and Ba) have been reported under normal pressure. In order to find other stable crystal structures, we have performed XN₆ (X = Ca, Sr and Ba) exploration under high pressure. We found that SrN₆ has a new P phase at a pressure of 22 GPa and an infinite nitrogen chain structure, and BaN₆ has a new C2/m phase at 110 GPa, with an N₆ ring network structure. Further, we observed that the infinite nitrogen chain and the N₆ ring network structure contain typical covalent bonds formed by the hybridization of the sp² and sp³ orbitals of N, respectively. It is found that both SrN₆ and BaN₆ are semiconductor materials and the N-2p orbital plays an important role in the stability of the crystal structure for P-SrN₆ and C2/m-BaN₆. 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 (XN₆, X = Ca, Sr and Ba).