High-Pressure FeNx: Stability, Phase Transition, and Energetic Characteristic

P (1) over bar -FeN4 with polymeric nitrogen chains has recently been synthesized by Bykov et al. at a temperature of 2000 K and pressure of 106.8 GPa. Upon pressure release, this material was recoverable until 22.7 GPa at room temperature, which disagreed with the theoretically predicted dynamical stability at ambient pressure. To clarify this discrepancy, we conduct the structure search of high-pressure FeNx using CALYPSO and calculate the pressure-dependent phase transition of P (1) over bar -FeN4 using a variable-cell double-ended surface walking method. The results show a quantitative trend of decreasing barrier upon pressure release and, more importantly, a switching of phase transition pathways at 15-20 GPa that reduces the barrier by more than one-half. In addition, the energetic characteristics of nitrogen-rich high-pressure FeNx compounds are evaluated. While P (1) over bar -FeN4 performs worse than RDX and HMX, P (1) over bar -FeN5, C2/m-FeN6, and Pnnm-FeN8 are found to be promising candidates for high-energy-density materials.