ON THE STRUCTURE FACTOR FOR COMMENSURATE MODULATED STRUCTURES

A superstructure may be looked upon as a commensurate modulated struct ure and be described through the superspace formalism. It can be studi ed using low- or high-symmetry superspace groups. The possibility of a dopting a high-symmetry description is related to the existence of at least two independent three-dimensional sections in the supercrystal w ith the same space-group symmetry. The usual structure-factor formula for commensurate structures [Yamamoto (1982). Acta Cryst. A38, 87-92] includes just one of these sections, thus allowing for a low-symmetry description only. A very simple generalization of this in the superspa ce formalism is presented, which allows for the use of either low- or high-symmetry superspace groups in the structural analysis. The reason s for a seemingly successful refinement found in the literature, makin g use of the high-symmetry description, are discussed.