Stable nitrogen-rich scandium nitrides and their bonding features under ambient conditions.

All-nitrogen salts have attracted extensive attention because of their unique chemical, physical properties, and potential applications as high-energy density materials. Using first-principles calculations and a particle swarm optimization structure search method, the pressure versus composition phase diagram of the Sc-N system is established. A new stable phase of C2/m-ScN5 with the intriguing 2D N106- is identified for the first time and we also found ScN3 with the P1[combining macron] structure which has been reported before. Under ambient conditions, both of them have high kinetic and thermodynamic stability with high energy density (2.40 kJ g-1 and 4.23 kJ g-1 relative to ScN and N2 gas). The analyses of chemical bonding pattern indicate that the nitrogen atoms in the N66- chains are connected by covalent bonds with a combined σ and π bond character. We also give a possible high-pressure experimental route to P1[combining macron]-ScN3 and C2/m-ScN5 at modest pressure. We expect that our theoretical research could encourage experimental realization in the future and contribute to the understanding of the bonding features of nitrogen chains.