Theoretical study of thermal stability of α″-Fe₁₆N₂ against other iron nitrides

α″-Fe₁₆N₂ has been investigated as one of promising candidates for environment-friendly magnets. While giant saturation magnetization has previously been experimentally observed in α″-Fe₁₆N₂, its magnetic anisotropy and structural stability leave room for improvement. Recent theoretical studies have considered alloying Fe₁₆N₂ with various elements to improve the magnetic properties and/or stability against decomposition. However, estimates of stability in particular are typically restricted to simple ground-state-energy comparisons, i.e. effectively taken at 0 K. For a more practical measure of stability, we therefore extend ground-state energies, obtained with the plane-wave density-functional theory code Quantum ESPRESSO, with appropriate empirical and/or statistical corrections to obtain free energies at arbitrary temperature. We then compare the stability of Fe₁₆N₂ against the neighboring phases in the Fe-N binary system, to estimate the range of temperatures at which it is stable. We compare against experimental observations of the Fe-N phase diagram.