Inter-sublattice ordering correlations and the geometrical locus approach to localized diffuse scattering

The fundamental tenet underlying the geometrical locus-cluster expansion ap proach to localized diffuse scattering has been re-examined. It is shown th at the theory, as it currently stands, can be successfully applied only to single disordered sublattice systems and has to be expanded when inter-subl attice ordering correlations exist. Such inter-sublattice ordering correlat ions give rise to cosine fringe contributions to the diffuse intensity whic h are either incommensurable with respect to the average structure reciproc al-lattice basis vectors or which are commensurable but with a repeat that is a multiple thereof Information as to inter-sublattice ordering correlati ons in the latter case then necessitates taking into account intensity vari ations from one reciprocal-space unit cell to the next and is inevitably ob scured by multiple scattering in the case of electron diffraction.