A New Measurement of Crystal Shapes for Absorption Corrections

New approaches to scaling data measured on a CCD diffractometer

New methods have been developed for scaling the intensities of diffraction reflections measured on a diffractometer with a CCD detector. The algorithm involves cyclic alternation of the refinement of a structure model from experimental data corrected for anisotropic effects with the refinement of anisotropic parameters from initial data for a fixed structure model. The first method consists of refining scale factors for the intensities after they have first been corrected for anisotropic effects of known physical nature (thermal diffuse scattering, absorption, extinctionetc.). It has been shown that the majority of the observed biases of intensities are caused by the characteristics of the CCD detector and the technique used for data collection on the diffractometer, rather than by the properties of the sample. The second (alternative) method of fixed scaling has been implemented here. This method does not require refinement of empirical scale factors but uses intensity correction maps obtained by averaging the results derived from several experiments with different crystals. The first map describes the non-uniformity of the response over the surface area of the detector. The second map accounts for distortions dependent on the setting angles of the goniometer. The methods were checked for data measured on an Oxford Diffraction Xcalibur Sapphire 3 CCD diffractometer. The proposed methods significantly improve the results of the refinement and make it possible to obtain a structure model that is reproduced in repeated investigations of samples of the same compound.