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M. Kurahashi
[Adv. X-ray Chem. Anal., Vol.26, pp.169-180, 1995]
Initial structure models are required for Rietveld refinements using powder diffraction data. In the case of structure analysis of organic compounds, approximate coordinates have usually been obtained by single crystal analysis routines. It is a tiresome step to collect a set of F(hkl). Accordingly, simulation technology was applied in order to get an initial structure model. Its flow is as follows: 1. The lattice parameters and the space group are determined from the powder diffraction data. 2. Molecular conformation is determined by MO, MM, MD methods (Fig. 1). 3. The molecular position and orientation are changed in the unit cell so as to obtain a model with the lowest lattice energy. 4. A powder pattern is calculated for the model structure, and a correct structure is selected from the comparison with the observed pattern. This method was applied for the crystal structure analysis of o-nitrobenzoic acid, which exhibits steric hindrance, and a starting model for Rietveld refinement was successfully obtained.
Fig. 1. Plot of heat of formation for various conformations