High-Pressure Crystallographic Experiments with a CCD-Detector

Abstract

As nowadays CCD detectors have become routine tools for X-ray diffraction structure determinations, they are increasingly available in X-ray laboratories and are also more often applied in high-pressure studies. For high-pressure experiments, when the sample crystal is enclosed in the diamond-anvil cell (DAC), the diffractometers equipped with CCD detectors have certain advantages and also some disadvantages, when compared to the diffractometers with point detectors. CCD-detectors are convenient, because they allow data collections to be carried out without prior determinations of the crystal orientation matrix (UB matrix). When a diffractometer with a point detector is used, the UB matrix must be determined before the data collection [1]. The determination of the UB matrix is often troublesome from a set of few reflections located with a point detector, while the CCD detector considerably facilitates this task: in most cases the reflections from the diamond anvils and of the DAC beryllium discs can be sorted out and then the unit cell of the sample crystal can be immediately found automatically from large data sets or by using procedures for visualizing the nodes of the reciprocal lattice; these procedures and 3dimmesional reflection profiles available from 2-D detectors are also very efficient for accessing the quality of the sample. Another advantage of CCD detectors is the speed of data collection, which is much quicker than the procedures of point-detector diffractometers measuring reflections one by one. CCD detectors also record all the reciprocal-space “volume”, not only the reflections. This information is indispensable for investigating phase transitions, structural disorder (diffused scattering), and for the inspection of background.