PHONON ANOMALIES IN BETA-PHASE NIXAL1-X ALLOYS

The electronic band structure of beta-phase Ni(x)Al1-x has been calculated using a first-principles linear combination of atomic orbitals method, and then fitted by the nonorthogonal empirical tight-binding method for the evaluation of electron-phonon matrix elements. A Kohn-like anomaly in the TA2 branch along the [110] direction is attributed to both strong electron-phonon interactions and Fermi-surface nesting. Other phonon anomalies near q = 0.55(1,1,1)pi/a and q = 0.6(1,0,0)pi/a in beta-phase Ni0.50Al0.50 are predicted. For the Ni0.625Al0.375 composition, the TA2 anomaly shifts to smaller wave vectors and the phonon is predicted to be soft at T = 0 K. Introducing a thermal smearing of the Fermi surface is enough to stabilize this mode, which occurs near (1/3, 1/3, 0)pi/a and is associated with the well-known martensitic transformation.