Structural phase transition and bonding properties of high-pressure polymeric CaN3

Alkaline-earth metal polynitrides can be used as a type of starting materials in the synthesis of polymeric nitrogen, which is used as a potential high-energy-density material. The structural evolutionary behaviors of nitrogen in CaN3 were studied at the pressure up to 100 GPa using a particle-swarm optimization structure search combining with density functional theory. Two new stable phases with P and C2/m space groups at the pressures of 26 and 60 GPa were identified for the first time. Throughout the stable pressure range, these two structures are semiconductors and consist of the N atoms in sp2-hybrid states. To the best of our knowledge, this is the first time an N6 chain is reported in case of alkaline-earth metal polynitrides. The stable polynitrogen compounds and polymeric nitrogen as high-energy-density materials have potential applications. The present results open a new possible avenue to synthesize high-energy-density polynitrogen.