Phonon anomalies and elastic constants of cubicNiAlfrom first principles

Abstract

The phonon dispersion relation of $\mathrm{NiAl}$ in the cubic $B2$ structure is calculated using first-principles density-functional perturbation theory with pseudopotentials and a plane-wave basis set. Anomalies are present in acoustic branches along three major symmetry directions: $\ensuremath{\Gamma}\text{\ensuremath{-}}X$, $\ensuremath{\Gamma}\text{\ensuremath{-}}M$, and $\ensuremath{\Gamma}\text{\ensuremath{-}}R$, with the positions being in excellent agreement with experiment. Analysis of the Fermi surface and the generalized susceptibility shows that these are Kohn anomalies. Overall, the computed phonon frequencies significantly decrease with increasing lattice parameter. This unusual sensitivity is attributed to a two-dimensional van Hove singularity in the electronic density of states near the Fermi level. The phonon dispersion is compared with that of $B2$ $\mathrm{NiTi}$, and the origin of phonon anomalies in the high-temperature phase is found to be different in the two systems.