Structure and properties of new high-pressure phases of Fe$_7$N$_3$

The structure and properties of high-pressure phases of iron nitrides Fe$_7$N$_3$ in the pressure range of $50$–$150$ GPa have been studied with ab initio calculations within the electron density functional theory. A new phase $Amm2$-Fe$_7$N$_3$, which is the most energetically favorable in the pressure range of $43$–$128$ GPa, has been found using the USPEX (Universal Structure Predictor: Evolutionary Xtallography) algorithms. It has been thermodynamically shown that another high-pressure phase $\beta$-Fe$_7$N$_3$ is isostructural to a similar phase of iron carbide. The elastic properties have been calculated for all modifications $\varepsilon$-, $\beta$-, and $Amm2$-Fe$_7$N$_3$ stable at high pressures.