The Incommensurate Crystalline Phase and Its Symmetry

Abstract

The term “incommensurate structure” has become the general name of a certain class of modulated structures. Strictly speaking it is an inappropriate indication, since “incommensurate” (in the sense of incompatible, or having no common measure) clearly should refer to at least two things and not to a single object such as a crystal structure. The two things, in this case, are the two elements of a modulated structure as defined, for instance, by Cowley (1): the basic structure on the one hand, and the periodic perturbation of this basic structure resulting in the modulated structure on the other hand. Hence the proper, though inconvenient term is: incommensurably modulated structures. We shall however stick to the incorrect but short term used in the title of this paper.