Michal Dušek, Jan Rohlíček, Václav Petříček
Structure analysis of powder samples is used for determination of atomic structure for compounds where single crystals are not available or hardly obtainable. A grain of powder contributes to diffraction pattern which is then composed of diffractions of many randomly oriented microcrystals. Structure determination from powder data is much more difficult than for single crystals putting strong limits to complexity of the investigated compounds. However, this field of crystallography is developing rapidly and its scope is growing with better solution methods and with increasing quality of powder diffractometers.
The method of powder diffraction will be developed in the Department of structure analysis since 2011 linked to the newly opened Laboratory of powder x-ray diffraction. Our aim is to offer possibility of structure determination from powder data to other research groups, similarly like it has been working for single crystal x-ray diffraction.
The development of powder structure analysis will be coupled with development of the program Jana2006. This crystallographic software contains excellent tools for Rietveld refinement of standard as well as modulated structures but it lacks support for the beginning stages of the diffraction experiment, like indexation of diffraction pattern and various ways to solve the phase problem. In this area practical experience is unreplaceable.
The powder diffractometer installed in the Laboratory of powder diffraction will use monochromatic Cu-Kalpha1 primary beam. This relatively rare equipment requires longer experimental time but offers better resolutin for the powder profile with less overlaps comparing to the classical primary beam with alpha2 component. Such data can be used for structure determination of modulated structures. During the first year of operation this possibility will be intensively tested.
The most difficult step of powder structure analysis is obtaining the rough structure model. For this purpose the support for powders available in the program Superflip will be further developed. We suppose that in the future the first structure estimation could be obtained from precession electron diffraction (see The project of Electron diffractometer), which is under development by the team of electron microscopy. A grain of powder sample may behave as a single crystal under electron beam and with help of precession electron diffraction its diffraction data can be obtained and its atomic structure can be roughly determined. The structure obtained from precession electron diffraction can be further refined against x-ray powder diffraction data to obtaine more precise results.
Another important topic is development of the Fundamental parameters, the method allowing for calculation of the powder profile shape solely from the knowledge of instrumental parameters. The method has not been developed for our type of diffractometer (Empyrean of Panalytical) and especially it has not been developed for capillary measurements which are the most interesting for the purpose of quantitative structure analysis.
In case of interest in measurement and calculation of crystal structures from powder data please do not hesitate to contact us!
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