High-Pressure Stability and Electronic Properties of Sodium-Rich Nitrides: Insights from First-Principles Calculations.

Abstract

Using first-principles structure search calculations, we investigated the phase stability of sodium-nitrogen (Na-N) compounds under high pressure. Our study reveals that increasing pressure promotes the formation of Na-rich nitrides, leading to the prediction of three previously unreported stoichiometries: Na₂N, Na₅N, and Na₈N. Notably, the electride Na₅N undergoes a pressure-induced structural transition from a P6/mmm to a P6₃/mmc phase. This transformation is characterized by spatial reorientation and redistribution of interstitial anionic electrons (IAEs). In the P6₃/mmc phase, IAEs adopt a zero-dimensional, triangular-like configuration, whereas in the low-pressure P6/mmm phase, they form an interconnected, graphene-like network. With increasing pressure, P6₃/mmc phase undergoes a transition from metallic to semiconducting behavior due to the increased interaction between sodium and IAEs. Additionally, C2/m Na₈N, featuring triangular- and ship-like IAEs, is predicted to exhibit superconductivity. Our findings provide new insights into the behavior and stability of Na-rich nitrides under high-pressure conditions.