Pressure-stabilized polymerization of nitrogen in alkaline-earth-metal strontium nitrides.

High-pressure technology can help us to obtain excellent materials. We have explored alkaline-earth-metal strontium nitrides under different pressures, theoretically. A variety of stable Sr-N structures were predicted by the structure searching method using CALYPSO code. Six new stoichiometries, SrN, SrN, SrN, SrN, SrN, and SrN, were predicted. And our calculation proved that all these compounds were stable existing under ambient pressure up to 100 GPa. A rich variety of poly-nitrogen forms appeared in the newly predicted SrN compounds, including four nitrogen polymerization forms: ranging from N, N, N, and N molecules, to zig-zag nitrogen chains and extended chains connected by puckered "N" rings. Significantly, the 1D extended polymeric chain of puckered "N" rings was firstly identified in the P1[combining macron]-SrN structure at 60 GPa. Another N-rich C2/c-SrN was stable only under the relatively high-pressure of 20 GPa, but this phase can be quenched under atmospheric pressure. The N-rich phase SrN maintained structural stability when the pressure reached 50-70 GPa. The delocalization of π electrons from N atoms was the principal cause for its metallicity in SrN. In this paper, our calculated results indicated that the energetic poly-nitrides in alkaline-earth-metal nitrides can be obtained by the high-pressure method.