Deformation paths and atomistic mechanism of B4 → B1 phase transformation in aluminium nitride

Two homogeneous deformation paths under uniaxial tension and shear are modeled for the wurtzite (B4) to rocksalt (B1) phase transformation of AlN by ab initio density functional theory. The B4 → B1 transition is a two-stage process along the deformation paths: an anti-parallel vertical movement of Al and N atoms along the crystallographic 〈0 0 0 1〉 axis, followed by a horizontal rearrangement of the relative positions of each type of atom. Each of these steps appears as a discontinuity in the tensile stress-strain curve. The shear path shows no discontinuity of the stress-strain curve in the first stage, while the second stage proceeds in a similar manner to the first stage of the tension path. No stress-free intermediate state is observed along the tension path owing to the sudden discontinuous lattice reconstruction, whereas along the shear path, a stress-free intermediate state is found at the end of the first stage, but it is intrinsically unstable because it is located at an unstable point of the energy-strain curve.