Ab initio based method to study structural phase transitions in dynamically unstable crystals, with new insights on the β to ω transformation in titanium

We present an approach that enables an efficient and accurate study of dynamically unstable crystals over the full temperature range. The approach is based on an interatomic potential fitted to ab initio molecular dynamics energies for both the high- and low-temperature stable phases. We verify by comparison to explicit ab initio simulations that such a bespoke potential, for which we use here the functional form of the embedded atom method, provides accurate transformation temperatures and atomistic features of the transformation. The accuracy of the potential makes it an ideal tool to study the important impact of finite size and finite time effects. We apply our approach to the dynamically unstable beta (bcc) titanium phase and study in detail the transformation to the low-temperature stable hexagonal omega phase. We find a large set of previously unreported linear-chain disordered (LCD) structures made up of three types of [111](beta) linear-chain defects that exhibit randomly disordered arrangements in the (111)(beta) plane.