Pressure-induced stability and polymeric nitrogen in alkaline earth metal N-rich nitrides (XN6, 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 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 P1̄ 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 P1̄-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).