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

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