SURFACE ALLOY FORMATION AND INTERDIFFUSION IN (ROOT-5X-ROOT-5)R27-DEGREES-YB AL(001) - A COMBINED LOW-ENERGY-ELECTRON DIFFRACTION AND X-RAYPHOTOELECTRON DIFFRACTION STUDY/

We have investigated the atomic structure of (root 5 x root 5)R27 degr ees-Yb/Al(001) by low-energy electron diffraction and angle-scanned fu ll-hemispherical X-ray photoelectron diffraction. Quantitative agreeme nt between the results from the two methods is obtained. We find that the Yb atoms occupy substitutional sites, thus removing every fifth su rface Al atom. The residual first-layer Al atoms are displaced by more than 0.5 Angstrom from their equilibrium position, in a way that resu lts in a quasi-twelve-fold Al coordination for the Yb atoms. An expans ion of the outermost substrate layer spacing and anomalously large vib rational amplitudes for the Al atoms of this strongly reconstructed fi rst substrate layer are determined. Furthermore, a small fraction of Y b atoms is identified to occupy second-layer sites in otherwise unreco nstructed surface patches. The comparison of the results obtained from the two techniques indicates the importance of a treatment of thermal vibrations going beyond the traditional Debye-Waller model in adsorba te X-ray photoelectron diffraction. (C) 1997 Elsevier Science B.V.