Obtainment of precise experimental data on diffractometers with 2D and point detectors
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Obtainment of Precise Experimental Data on Diffractometers with 2D and Point Detectors A. P. Dudka Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiœ pr. 59, Moscow, 119333 Russia e-mail:[email protected] Received November 17, 2006
Abstract—A measurement method based on the use of diffractometers with a point detector is proposed that makes it possible to reduce the measurement time by a factor of about 2 in comparison with the conventional experiment. Measures are proposed that allow one to increase the measurement precision for diffractometers with a 2D CCD detector. Refinement of the model of independent atoms from the experimental data obtained by these methods is characterized by consistency between the experimental and calculated structure factors at a level of 1–1.5%. A further significant decrease in refinement R factors in going to the multipole model indicates that the data precision is sufficient for studying the charge density in crystals. PACS numbers: 61.66.Fn, 71.20.-b DOI: 10.1134/S1063774508040275
INTRODUCTION The range of application of diffractometers with 2D detectors in structural investigations is constantly increasing. The use of these instruments makes it possible to reduce the time of experiment in comparison with the measurements based on diffractometers with point detectors. Nevertheless, point detectors are still in use because they provide high precision of structural results. During the transition period, until point detectors exhaust their resource, there is the opportunity of performing studies aimed at comparing and improving the possibilities of analysis with diffractometers of both types. The methods proposed here have been developed during investigations of the structure of three groups of crystals with different symmetry. The structural results of desired precision and completeness for these crystals were obtained owing to the use of improved analytical methods. The analytical technique based on the use of point detectors has been thoroughly developed [1, 2]. The main drawback of this technique is the long measurement time and, as a result, possible systematic errors of data due to the drift of the equipment parameters, sample decomposition, etc. An advantage of point detectors is the flexible access to measurement of each reflection. Potentially, this possibility provides adequate influence of each reflection on the procedure of refining the crystal model, which makes it possible to obtain structural results of high relative precision. The technique of the analysis based on 2D detectors has been discussed much less; the emphasis was on the correction of systematic errors [3, 4] and data process-
ing [5]. Drawbacks of 2D detectors, operating in the standard mode specified by the manufacturer, are as follows: (i) the obtained data set includes reflections that were measured only once and does not include measured symmetrically equivalent reflections (below, such reflections are referred to as single), and (ii) monitoring of t
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