Evaluierung der harmonischen Näherung durch Berechnung anisotroper Auslenkungsparameter

  • Evaluation of the harmonic approximation based on the calculation of anisotropic displacement parameters

Mroz, Damian Patrick; Dronskowski, Richard (Thesis advisor); Raabe, Gerhard (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

This thesis deals with the evaluation of the harmonic approximation based on anisotropic displacement parameters. It is possible to obtain these parameters with the help of the DFT method in conjunction with phonon calculations. These ADPs can potentially be used with regard to crystallography. One example of this is the anisotropic refinement of hydrogen based on XRD results, which is generally not possible. Therefore theory can be used to help with the task of structure refinement. In contrast to this the experimental treatment of heavy atoms is relatively easy. However, the theoretical prediction of such ADPs can potentially be problematic, because small frequencies in the case of heavy atoms may lead to huge deviations in the results. Small changes may already significantly affect the calculated ADPs. Therefore, theory and experiment are complementary to each other. Nonetheless, this thesis aims to study the potential problems of both of them. It is conceivable that problems with heavy atoms may not be as pronounced as expected. It is possible that the experiment, which serves as the benchmark for the theory, may suffer from problems. Such problematic areas could be absorption problems or insufficient redundancy of the measurements, which can result in systematic or statistical errors. It may well be possible that the quality of the harmonic approximation is already sufficient. In order to ensure a homogeneous evaluation of theoretical ADPs, the experiment is extended with synchrotron measurements of two isomorphous molecular crystals to enable a mutual evaluation of theory and experiment. Furthermore, these synchrotron measurements are also used to obtain temperature resolved ADPs, which can be used to characterize the range for which theory predicts satisfactory anisotropic displacement parameters. Moreover, a new quantitative tool for the correspondence between theoretical and experimental direction of thermal movement is developed. The aim is to develop an intuitive tool that can be used to compare the results from theory and experiment. In summary, this thesis tries to illustrate the quality of the harmonic approximation from different points of view.

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