The centrosymmetric-noncentrosymmetric ambiguity: some more examples

Four more examples are provided to emphasize the extreme difficulty in deciding, by diffraction methods, whether a crystal structure is centrosymmetric or only approximately so. In these examples, earlier workers described and refined structures in noncentrosymmetric space groups; refinements in the corresponding centrosymmetric space groups, based on the original data, lead to improved results. In one case, apparent violations of systematic absences seem to preclude the centrosymmetric description; however, other evidence - in particular, improved agreement for the very weak reflections (which are the most sensitive to the centrosymmetric non-centrosymmetric ambiguity) - suggest that the space-group violations might be spurious. In any event, the moral is clear: extreme caution is needed when attempting to derive a noncentrosymmetric description of a closely centrosymmetric structure.