High-pressure phase transitions of nitinol NiTi to a semiconductor with an unusual topological structure

Systematic ab initio structure simulations have been used to explore the high-pressure behavior of nitinol (NiTi) at zero temperature. Our crystal structure prediction and first-principles calculations reveal that the known B19 phase is dynamically unstable, and an orthorhombic structure (Pbcm) and a face-centered-cubic B32 structure (Fd3m) become stable above ∼4 and 29 GPa, respectively. The predicted, highest-pressure, B32 phase is composed of two interpenetrating diamond structures, with a structural topology that is quite distinct from that of the other phases of NiTi. Interestingly, the B32 phase shows an unusual semiconducting characteristic as a result of its unique band structure and the nature of 3d orbitals localization, whose expected synthesis pressure is accessible to current experimental techniques.