DFT Study of Lattice Dynamics and Phase Transitions of Mg2NA (A = F, Cl, Br, I) Compounds in Antichalcopyrite and Anti-LiFeO2 Structures

Abstract

Structural parameters of Mg₂NA (A = F, Cl, Br, I) crystals with antichalcopyrite and anti-LiFeO₂ structures are determined and their vibrational spectra are studied using the density functional theory (DFT). It is shown that that the chloride, bromide, and iodide spectra contain features in the form of acoustic branches with imaginary frequencies characteristic for unstable structures. For Mg₂NF, the energetically optimal path of the structural transition to the stable state is calculated. The transition path contains additional intermediate minima, for which symmetry and parameters of the corresponding crystal structures are determined. On this basis, a mechanism of a structural phase transition to Mg₂NF is proposed.