Partial disorder and leed diagrams

A241 Surface Science 129 (1983) 301-311 North-Holland Publishing C o m p a n y

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PARTIAL DISORDER AND LEED DIAGRAMS I. The S / P t ( I l O ) system V. M A U R I C E Laboratoire de Physico-Chimie des Surfaces, ERA 313, Universit~ Pierre et Marie Curie, ENSCP, 11 Rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France

and J.J. L E G E N D R E and M. H U B E R Laboratoire de Physico-Chimie et Structures des Solides Imparfaits, 1_,4 302, Universit~ Pierre et Marie Curie, ENSCP, 11 Rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France Received 28 December 1982; accepted for publication 24 March 1983

The LEED diagram observed for S/Pt(110) in the 4 / 1 2 - 5 / 1 2 coverage range shows a small coincidence unit mesh with some zero intensity spots. It can be interpreted by partial disorder and occupancy modulations. Two different methods of investigation have been used to get a structure model. The major part of this model is made of ordered structural elements of the configurations corresponding to the lower and higher concentrations and belonging to a single preponderant antiphase. The remaining part is made of atoms belonging to the other antiphases which are disordered with respect to the preponderant region. A unique system of modulations is obeyed in all regions, which combine to produce the zero intensities of the missing spots.

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Surface Science 129 (1983) 312-326 North-Holland Publishing C o m p a n y

PARTIAL DISORDER AND LEED DIAGRAMS 11. The S / A g ( 1 0 0 ) and S / C u ( 1 0 0 ) systems V. M A U R I C E Laboratoire de Physico-Chimie des Surfaces, ERA 313, Universitb Pierre et Marie Curie, ENSCP, 11 Rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France

and J.J. L E G E N D R E and M. H U B E R Laboratoire de Physico-Chimie et Structures des Solides lmparfaits, I_,.4 302, Universitk Pierre et Marie Curie, ENSCP, 11 Rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France Received 28 December 1982; accepted for publication 24 March 1983

At high concentrations, sulfur monolayers adsorbed on the (100) faces of Ag and Cu give very special LEED diagrams. Structure models have been calculated directly with a computerized trial and error method. They are partially disordered and show two systems of occupancy modulation which appear simultaneously: these systems are not independent. Incoherent domains at right angles are necessary to explain the fourfold symmetry.