Surface-phonon dispersion of NiAl(110)

We have measured the surface-phonon dispersion for the (110) face of the ordered alloy NiAl using high-resolution electron-energy-loss spectroscopy (EELS) with {Δ}E=20--45 cm^-1 both in the [100] ({Γ}{}--Y{}) and in the [110] ({Γ}{}--X{}) directions. To identify and fit the observed branches, lattice-dynamical calculations employing a Born--von K\'arm\'an model up to third-nearest neighbors were performed. Good agreement between the experimental and calculated dispersion curves is obtained when the force constant between first-layer nickel atoms and second-layer aluminum atoms is increased by 50% and the force constant between first-layer aluminum atoms and second-layer nickel atoms is decreased by 20%. These force-constant changes indicate a surface rippling, where first-layer nickel atoms are displaced inwards and first-layer aluminum atoms are displaced outwards. This is consistent with a structural model based on low-energy electron diffraction (LEED) I-V data of Noonan and Davis [Phys. Rev. Lett. 59, 1714 (1987)] and a medium-energy ion-scattering (MEIS) study by Yalisove and Graham [Surf. Sci. 183, 556 (1987)].