Ab initio study of pressure stabilized NiTi allotropes: Pressure-induced transformations and hysteresis loops

Changes in stoichiometric NiTi allotropes induced by hydrostatic pressure have been studied employing density functional theory. By modeling the pressure-induced transitions in a way that imitates quasistatic pressure changes, we show that the experimentally observed B19 ' phase is (in its bulk form) unstable with respect to another monoclinic phase, B19 ''. The lower symmetry of the B19 '' phase leads to unique atomic trajectories of Ti and Ni atoms (that do not share a single crystallographic plane) during the pressure-induced phase transition. This uniqueness of atomic trajectories is considered a necessary condition for the shape memory ability. The forward and reverse pressure-induced transition B19 ' <-> B19 '' exhibits a hysteresis that is shown to originate from hitherto unexpected complexity of the Born-Oppenheimer energy surface.