Phonon anomalies in β-phaseNixAl1−xalloys

The electronic band structure of β-phase ${\mathrm{Ni}}_{\mathit{x}}$ ${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$ 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 ${\mathrm{TA}}_2$ 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)π/a and q=0.6(1,0,0)π/a in β-phase ${\mathrm{Ni}}_{0.50}$ ${\mathrm{Al}}_{0.50}$ are predicted. For the ${\mathrm{Ni}}_{0.625}$ ${\mathrm{Al}}_{0.375}$ composition, the ${\mathrm{TA}}_2$ 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)π/a and is associated with the well-known martensitic transformation.