Prediction of structure of CON and NaN crystals under high pressure.

Structure of CON and NaN crystals under pressure is predicted using evolutionary algorithm and

density functional theory (DFT). The found structures complement results of the previous

structure searches. The structure of CON2 crystal at 40 GPa is made of eight-membered rings of

singly bonded CON2 molecules in a three-dimensional framework structure in the space group

of P43. It has high mass density (3.98 g/cc) comparable with that of cubic gauche nitrogen and

high content of nitrogen atoms forming a spiral perpendicular to the eight member rings. The

structure is in a good agreement with X-ray diffraction experiments. Metallic compounds

Na3(N2)4 at ~28 GPa, NaN2 at 4 GPa and NaN5 at 50 GPa are found to be thermodynamically

stable according to the convex hull calculations. Calculated phonon dispersions indicate

dynamical stability of the systems, calculated Raman active frequencies of the compounds are

compared with experiments.