Theoretical Estimate of Thermal Stability Range of α″–Fe₁₆N₂ Within the Iron Nitride Binary PhaseDiagram

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, 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 (DFT) 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 and phase combinations in the Fe-N binary system within the Compound Energy Formalism, to estimate the range of temperatures at which it is stable. Depending on choice of correction terms, Fe₁₆N₂ ay be predicted to persist well above 800 K with simple corrections, or limited to below ~ 450 K yielding to α–Fe + ε–Fe₃N at higher temperatures with more advanced terms. We compare against experimental observations of formation enthalpies and the Fe-N phase diagram, and discuss the effect and relative accuracy of different correction terms.