高压下γ′-Fe_4N晶态合金的声子稳定性与磁性

Phonon stability and magnetism of γ′-Fe_4N crystalline state alloys at high pressure

利用基于密度泛函理论的第一性原理研究了高压下有序晶态γ′-Fe4N合金的晶格动力学稳定性与磁性.对比没有考虑磁性的γ′-Fe4N的声子谱,得出压力小于1 GPa时,自发磁化诱导了铁磁相γ′-Fe4N基态晶格动力学稳定.压力在1.03—31.5 GPa时,Σ线上的点(0.37,0.37,0)、对称点X和M上相继出现了声子谱软化现象.压力在31.5—60.8 GPa时,压致效应与自发磁化对诸原子的作用达到了稳定平衡,表现出了声子谱稳定.压力大于61.3 GPa时,随着压力的增大压力诱导体系动力学不稳定性越强.通过软模相变理论对于γ′-Fe4N,在10 GPa下的声学支声子的M点处软化现象的处理,发现了动力学稳定的高压新相P2/m-Fe4N.压力小于1 GPa时高压新相P2/m-Fe4N是热力学稳定的相,且磁矩与γ′-Fe4N的磁矩几乎相同.2.9—19GPa时,P2/m相的焓比γ′相的焓小,基态结构更稳定.大于20 GPa时,两相磁矩几乎相同.

By using projection augmented plane wave method（PAW） and based on the density functional theory, the stability of lattice dynamics and the magnetism of ordered crystalline alloy γ′-Fe4 N are studied at high external pressures. In comparison with the phonon spectrum of γ′-Fe4 N without considering the spin-polarization, it is found that the groundstate lattice dynamics stability of the ferromagnetic phase γ′-Fe4 N is induced by the spontaneous magnetization at pressures below 1 GPa. The phonon spectra at point（0.37, 0.37, 0） in line Σ, points X and M become softening at pressures between 1.03 and 31.5 GPa. The pressure-induced effect and the spontaneous magnetization effect on the atoms reach a stable equilibrium state at the pressures between 31.5 and 60.8 GPa, which result in the phonon spectrum stability. As the pressure exceeds 61.3 GPa, the system becomes more instable dynamically with the increase of the external pressure. The softening at point M of the acoustic phonon is treated by the soft-mode phase theory at 10 GPa, and a new dynamic stability high-pressure phase with a space group of P2/m is found. This new phase is thermodynamically stable and possesses the same magnetic moments as that of γ′-Fe4 N at pressures below 1 GPa. The enthalpy value of the phase P2/m is less than that of phase γ′at the pressures between 2.9 and 19 GPa, therefore its ground-state structure is more stable. As the pressure exceeds 20 GPa, both phases possess almost the same magnetic moments.