B4−B1phase transition of GaN under isotropic and uniaxial compression

Abstract

The question raised recently about the mechanisms of the pressure-induced $B$4-$B$1 transition of GaN is investigated using a density functional-based metadynamics method. A homogeneous deformation path through a tetragonal intermediate structure is found to be energetically favorable under hydrostatic pressure conditions. This transition path is initialized by the soft phonon modes that appear along the transverse acoustic branches near the transition point. However, under certain nonhydrostatic pressure conditions an elastic instability replaces the phonon instability to become a dominant mechanism for the structural transformation of GaN. The phase transition would avoid the tetragonal path when a uniaxial stress is applied on the $c$ axis but follows a different path through a hexagonal intermediate structure. The selection of the transformation mechanisms, determined by specific stress conditions applied, highlights the important role the competition between phonon and elastic instabilities plays in the reconstructive phase transitions.