Spontaneous magnetization-induced phonons stability in γ′-Fe4N crystalline alloys and high-pressure new phase

Highlights: • A new dynamic stability high-pressure phase in Fe₄N with space group of P2/m is found • The Fe atom of γ-Fe is at lattice dynamics unstable equilibrium position. With the interstitial N atom entered into body center, the magnetic moments of Fe atom of γ′-Fe₄N redistribution by the system of spontaneous magnetization, and makes the phonon spectrum stable. • The spontaneous magnetization induces the dynamic stability of ferromagnetic phase γ′-Fe₄N less than 1 GPa. The stability of lattice dynamics and the magnetism of the ordered γ′-Fe₄N crystalline alloy at high pressures were studied by first-principle calculations based on density-functional theory. The dynamical stable new phase P2/m-Fe₄N at high pressures was found by conducting the softening phenomenon at the point M (0.5 0.5 0) of the acoustic phonon at 10 GPa in the γ′-Fe₄N via soft-mode phase transition theory. Compared to the phonon spectrum of γ′-Fe₄N without considering electronic spin polarization, the ground-state lattice dynamical stability of the ferromagnetic phase γ′-Fe₄N is induced by the spontaneous magnetization at pressures below 1 GPa. However, P2/m-Fe₄N is more thermodynamically stable than γ′-phase at pressures below 1 GPa, and the magnetic moments of the two phases are almost the same. The ground-state structure of P2/m phase is more stable than that of γ′-phase in the pressure range from 2.9 to 19 GPa. The magnetic moments of the two phases are almost the same in the pressure range from 20 to 214 GPa, but the ground-state structure of γ′-phase is more stable than that of P2/m phase in the pressure range from 143.8 to 214 GPa. On the contrary, the ground-state structure of P2/m phase is more stable when the pressure is above 214 GPa. In the pressure range from 214 to 300 GPa, the magnetic moment of P2/m phase is lower than that of γ′-phase, and the magnetic moments of the two phase tend to be consistent when the pressure exceeds 300 GPa.