High-Pressure FeN<i>_x</i>: Stability, Phase Transition, and Energetic Characteristic

P1̅-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 P1̅-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 P1̅-FeN4 performs worse than RDX and HMX, P1̅-FeN5, C2/m-FeN6, and Pnnm-FeN8 are found
to be promising candidates for high-energy-density materials.