Bulk and surface vibrational modes in NiAl.

Abstract

We have measured the phonon dispersion curves of bulk NiAl along the three major symmetry directions by neutron scattering, and fitted the results with force models extending to third- and fourth-nearest neighbors. Our electron-energy-loss experiments show that dipole-active vibrational modes occur in the gap between the bulk acoustical and optical modes for the (110) and (111) surfaces. We have done extensive slab calculations of the vibrational modes at the surface-Brillouin-zone center for these two faces and for NiAl(001). The calculations indicate that the atomic motion for the gap modes is correlated with the bulk properties. Adjustment of the interplanar force constants in directions intuitively consistent with experimental results for the surface relaxations can shift the calculated frequencies into agreement with experiment.