The characterization of modulated structures via their diffraction patterns

The existence of a modulated structure is signified in reciprocal space by the existence of weak extra features in addition to the strong Bragg reflections of the unmodulated parent structures. These weak extra features may be sharp incommensurate satellite reflections and/or a diffuse intensity distribution. The extraction of information as regards the character of such modulated structures from a study of their diffraction patterns is reviewed. In Section 2, the structural parameters defining the deviation of modulated structures from their corresponding parent phases is discussed. In Section 3, the observation and interpretation of satellite extinction conditions is discussed. In Section 4 and 5, the qualitative features often present in the diffraction patterns of modulated structures are related to structure factor expressions.