X-ray data collection from mineral crystals by means of a position-sensitive detector: Advantages and disadvantages

In order to check for the accuracy of X-ray diffracted data collected with an area-detector diffractometer (FAST-Nonius), we have carried out several data collections on a good-quality pyrope crystal (space group 1a3d; a = 11.479 Å) under different experimental settings and compared the results with those obtained with the same crystal mounted on a conventional Philips PW1100 diffractometer. Several parameters have been tested (detector gain, crystal-to-detector distance, frame width, integration time per image, beam intensity, shoebox size and re-measuring of overflow reflections), and four critical features of the system have been identified: the low thermal stability of the detector, its narrow dynamic range, the importance of the detector-to-crystal distance and the integration of the diffracted intensities. We are now able to select the best experimental settings in order to obtain a refinement from FAST diffraction data good as that from Philips data, in terms of Rsym, Robs and standard deviation of the refined parameters.