MEAN-SQUARE DISPLACEMENTS OF SURFACE ATOMS - C(2X2) OVERLAYERS ON NI(100)

We present lattice-dynamical calculations of the mean-square displacements of the adsorbate and first-layer substrate atoms for c(2×2) S and O overlayers on Ni(100). To be consistent with available structural information and to reproduce the observed surface-phonon dispersion data on these systems, surface force constants between Ni atoms are chosen to be different from the value in the bulk. It is found that for sulfur atoms the mean-square displacement in the plane parallel to the surface is larger than that along the surface normal. The opposite is the case for the oxygen atoms. The underlying Ni atoms in the two systems also display dissimilar anisotropies in the three orthogonal directions. We have examined the relative strengths of the mean-square relative displacements between the adsorbate and the first- and second-layer substrate atoms for both systems and explored the effect of the variation of the surface force constants on these quantities. These results have been brought in contact with available surface extended x-ray-absorption fine-structure data on these systems. © 1990 The American Physical Society.