Phase transformations of Fe$_3$N-Fe$_4$N iron nitrides at pressures up to $30$ GPa studied by in situ X-ray diffractometry

The parameters of the equation of state of the stable $\varepsilon$-Fe$_3$N$_x$ (where $x = 0.8$) nitride in the Fe–N system have been determined at pressures up to $30$ GPa and temperatures up to $1273$ K. The parameters $V_0=81.48(2)$ Å$^3$, $K_T=162(3)\,$GPa, $K_T=4.0=1.66(2)$, $\gamma_0=555\,$K, and $q = 1$ have been determined for $\varepsilon$-Fe$_3$N$_{0.8}$ by the approximation of the $P$-$V$-$T$ data with the Vinet equation of state and the thermal parameters within the Mie–Gürneisen–Debye formalism. No anomalous change in the volume of the cell owing to possible magnetic transitions has been revealed. The instability of Fe$_4$N at high pressures and Fe$_3$N in the presence of a deficit of nitrogen in the system has been established. The stable phase in the temperature range of $300$–$673$ K and the pressure range of $20$–$30$ GPa is $\varepsilon$-Fe$_3$N rather than $\varepsilon$-Fe$_3$N$_{0.8}$.