ELECTRON-PHONON INTERACTIONS AND THE PHONON ANOMALY IN BETA-PHASE NITI

The electronic structure of beta-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)pi/a, which is responsible for the premartensitic phase transition in beta-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 tum cause a hardening of the phonon anomaly.