Pressure-Stabilized New Phase of CaN_4

We propose a new CaN_4 high pressure structure with the P2_1/m space group. The P2_1/m-CaN_4 structure is constituted by the infinite armchair N-chain. The dynamical stability and mechanical stability are verified by the calculations of phonon dispersion curves and elastic constants. The enthalpy difference calculation shows that the P2_1/m phase is more stable than the reported P4_12_12 phase. The advantaged properties of P2_1/m-CaN_4, such as high nitrogen content (58.3%) and low polymerization pressure (18.3 GPa), allow it to be a potential high energy material. Band structure calculation shows that the P2_1/m-CaN_4 structure is a metallic phase. The nonpolar covalent single N–N bond is a sigma bond. The charge transfer between the Ca and N atoms results in an ionic bond interaction.