Theoretical study of $\gamma'$-Fe$_4$N and $\varepsilon$-Fe$_x$N iron nitrides at pressures up to 500 GPa

The parameters of equations of state of stoichiometric and nonstoichiometric phases of the $\gamma'$-Fe$_4$N, $\varepsilon$-Fe$_3$N$_{0.75}$, $\varepsilon$-Fe$_3$N, $\varepsilon$-Fe$_3$N$_{1.25}$, and $\varepsilon$-Fe$_3$N$_{1.5}$ iron nitrides in a pressure range up to 500 GPa have been determined using ab initio calculations. The points of the sharp drop and disappearance of the magnetic moment on iron atoms have been found. It has been shown that certain changes in the magnetic moment are accompanied by a change in the volume of a unit cell of the nitrides. The calculated parameters of equations of state demonstrate that the compressibility of both magnetic and nonmagnetic iron nitrides decreases monotonically with an increase in the content of nitrogen.