Electron-phonon interactions and the phonon anomaly in β-phase NiTi

The electronic structure of {β}-phase NiTi has been calculated using a first-principles linear-combination-of-atomic-orbitals method. The resulting band structure was fitted with a nonorthogonal tight-binding Hamiltonian from which electron-phonon matrix elements were evaluated. The soft phonon near Q₀=(2/3, 2) / 3 ,0){π}/a, which is responsible for the premartensitic phase transition in {β}-phase NiTi, is found to arise from the strong electron-phonon coupling of nested electronic states on the Fermi surface. Thermal vibrations and changes in electronic occupation cause a smearing of the nested features, which in turn cause a hardening of the phonon anomaly.