SURFACE PHONON-DISPERSION OF ALLOY SURFACES

The dispersion of surface phonons for different alloy surfaces has been studied using electron energy loss spectroscopy with a resolution DELTA-E = 20 - 45 cm-1. The dispersion of several phonon branches has been determined for the (110) surface of the ordered alloy NiAl(110). Good agreement between the experimental and calculated data can be obtained after an adjustment of force constants at the surface. These force constant changes and a cross section analysis indicate a rippled relaxation. Even quantitative agreement is obtained with a structural model based on LEED and ion scattering data. The dispersion of the Rayleigh wave has been determined for the (100) surface of the disordered alloy Ni90Pt10. This mode shows a strong broadening which is presumably an intrinsic effect of the disordered alloy. The frequency of the vibration is lower than the Rayleigh wave on Ni(100) over the entire Brillouin zone.