First-principles calculations of thermal properties of the mechanically unstable phases of the PtTi and NiTi shape memory alloys

The design of shape memory alloys (SMA) demands detailed knowledge of their thermodynamic properties and of the underlying atomic-level mechanisms governing their shape-memory behavior. This knowledge is traditionally obtained via atomistic simulations, but these systems have so far resisted such efforts due to the presence of phonon instabilities and associated spontaneous symmetry breaking. In this study, we investigate the thermodynamics of PtTi, a novel and promising SMA (and of the related better-known NiTi SMA, for comparison purposes) using recently developed first-principles computational method specifically designed to tackle this issue. The method efficiently explores the system's potential energy surface by discrete sampling of local minima, combined with a continuous sampling of the vicinity of these local minima via a constrained harmonic lattice dynamic approach. Our calculations provide a complete and atomic-level-based model for these compounds' free energy and shed some light on the ongoing search for the precise structure of dynamically stabilized high temperature phases in SMAs.