COORDINATION ARRANGEMENTS IN SURFACE RECONSTRUCTIONS OF TRANSITION METALS STUDIED BY LOW-ENERGY ELECTRON DIFFRACTION. W. Liu, K. A. R. Mitchell, D. T. Vu, Y. M. Wang, K. C. Wong, Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1

The interaction of electronegative elements with well-characterized transition metal single- crystal surfaces can yield a wide variety of structural arrangements which may be studied by diffraction techniques. The most detailed information is from low-energy electron diffraction, particularly in the tensor LEED approach. For example, the types of structure formed with electronegative atoms like O, N, S or P can correspond to: (i) simple chemisorption, where electronegative atoms form chemical bonds to a relaxed, but not dramatically changed, metal surface; (ii) underlayer absorption; (iii) incipient surface compound formation, where a small number of compound-type layers bond to the metal substrate; or (iv) an independent adsorbate- induced surface reconstruction. A review will be made of some representative examples of structures recently determined on low-index surfaces of copper, rhodium, zirconium and palladium, including some discussion of reasons for particular choices of structure. Completely new structures are being uncovered. For example, P on the Rh(111) surface gives a reconstruction which involves a packed arrangement of nearly co-planar Rh pentagons and triangles on the surface. In this extremely stable structure each P atom, which is also essentially co-planar with the topmost metal layer, has an identical environment and bonds to eight neighbouring Rh atoms.