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 P1̄ phase at a pressure of 22 GPa and an infinite nitrogen chain structure, and BaN(6) has a new C2/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 P1̄-SrN(6) and C2/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).