LEED STRUCTURE-ANALYSIS OF P(2X2)S/NI(100)

With adsorption of sulfur on Ni(100) a streaky p(2 × 2) LEED pattern appears at coverage θ = 0.25. In spite of the corresponding existence of some disorder in the adlayer a LEED structure analysis of satisfying quality could be performed (Pendry R-factor R = 0.21). Sulfur adsorbs in the hollow site with an adsorption height of 1.25 ± 0.03 Å. The topmost substrate layer spacing is changed from 1% contraction as derived for the clean surface to 0.5 ± 1% expansion. The nickel atoms below the adsorbed sulfur are shifted down by 0.07 ± 0.05 Å whereas the other atoms of the second substrate layer are not affected. The results are discussed to be in line with those obtained recently for the c(2 × 2)S structure and the similar superstructures observed with adsorption of oxygen. The observation of some unavoidable disorder in the p(2 × 2)S layer is discussed and contrasted to the similar but well-ordered phase of p(2 × 2)O/Ni(100) investigated earlier. The differences between the two systems are interpreted to be due to the different strength of interaction of the adsorbates with the substrate, in agreement with the details of the structural results. © 1990.