A new type of constraint in the maximum-entropy method using ambiguous phase information from anomalous-scattering powder data

Anomalous scattering of X-rays at a synchrotron source can be used for the ab initio structure determination of unknown crystal structures using only powder diffraction data. For noncentrosymmetric crystals, the phases of str ucture factors can only be determined with a remaining ambiguity, when one chemical element is used as resonant scatterer. A corresponding additional constraint function has been built into an enhanced version of the program MEED, so that now all types of information gained from an anomalous-scatter ing powder diffraction experiment can be used in a maximum-entropy calculat ion of the electron-density distribution: phased reflections, unphased refl ections, intensities of groups of overlapping reflections, and now also ref lections with a remaining ambiguity in the phase. This is important for pra ctical use, since a lot of information is already lost in the powder diagra m compared with single-crystal datasets and it is essential to use all rema ining information. The new constraint is demonstrated with the structure of Cu5Zn8.