Structure of incommensurately modulated chromium pyrophosphate studied by Maximum Entropy Method (MEM)

A general discussion is given of the Maximum Entropy Method (MEM) as a method to determine the electron density in superspace from X-ray diffraction data. The MEM has been used to determine the electron density in superspace of incommensurately modulated chromium pyrophosphate from X-ray diffraction data published by Palatinus et al. [Acta Crystallogr. B 62, 556]. Chromium pyrophosphate, Cr(2)P(2)O(7), contains ordered regions (83% of the volume) and regions with disorder. Analysis of the MEM density has allowed us to determine the displacive modulation functions within the ordered regions. The disordered regions can be described as the alternate occupation of two conformations of the pyrophosphate unit and two positions of the chromium atom. Both the conformations and the occupational probabilities of them depend continuously on the phase of modulation t.