- Modulation of Incommensurately Modulated Structures Studied by the Maximum Entropy Method (2011)
- In this thesis, modulated properties of incommensurately modulated crystals are studied by the Maximum Entropy Method (MEM). This was done exemplarily on the compounds Rubidium tetrachlorozincate and Chromium pyrophosphate. To do so, the MEM derived (3+1)-dimensional superspace electron density was analysed to receive information on atomic positions and their modulation, and structure refinement by the method of least-squares and interpretation of the difference-Fourier maps were performed to better describe the atomic displacement parameters (ADPs) and to improve the applied structural models. All the MEM calculations are done by the computer program BayMEM. The analysis of this MEM reconstructed density map is done by the computer program EDMA. Our results suggest that the modulation of harmonic ADPs, anharmonic ADPs and its modulation form an intrinsic part of incommensurately modulated structures. We have shown that with a data set of certain resolution and satellite reflections of higher order, the inclusion of modulation of harmonic ADPs, the anharmonic ADPs and the modulation of anharmonic ADPs can significantly improve the fit of the structure model to the diffraction data. Such model then better represents the true nature of the structure under investigation.