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, Sr₂N₃, SrN₂, SrN₃, SrN₄, and SrN₅, 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_x compounds, including four nitrogen polymerization forms: ranging from N₂, N₃, N₄, and N₅ molecules, to zig-zag nitrogen chains and extended chains connected by puckered "N₆" rings. Significantly, the 1D extended polymeric chain of puckered "N₆" rings was firstly identified in the P1[combining macron]-SrN₃ structure at 60 GPa. Another N-rich C2/c-SrN₄ was stable only under the relatively high-pressure of 20 GPa, but this phase can be quenched under atmospheric pressure. The N-rich phase SrN₅ 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 SrN₅. 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.