Application of x-ray direct methods to surface reconstructions: The solution of projected superstructures

The projections of surface reconstructions are normally solved fi om the interatomic vectors found in two-dimensional Patterson maps computed with the intensities of the in-plane superstructure reflections. Since for difficult reconstructions this procedure is not trivial, an alternative automated one based on the "direct methods" sum function [Rius, Miravitlles, and Allmann, Acta Crystallogr. A52, 634 (1996)] is shown. It has been applied successfully to the known c(4 X 2) reconstruction of Ge(001) and to the so-far unresolved In0.04Ga0.96As (001) p(4 X 2) surface reconstruction. For this last system we propose a modification of one of the models previously proposed for GaAs(001) whose characteristic feature is the presence of dimers along the fourfold direction.