Constraints, 'order' and new types of modulated phases

The strongest crystal chemical constraints are usually local and fix local parameters such as bond lengths, angles and even stoichiometries (although not necessarily orientations) in local co-ordination polyhedra. Surprisingly often, however, constraints of this sort are insufficiently severe or long range enough to lead to well-defined, conventional 3-d crystalline order. Instead quite distinct new types of"modulated structure,commonly associated with the existence of sharp, highly structured, characteristic diffuse intensity distributions, can result.