A Guide to the Determination of Operational Parameters in the Conventional X-ray Diffractometry of a Small Single Crystal

Abstract

There are two important operational parameters in conventional X-ray diffractometry of small crystals-scan range and detector aperture size. These parameters depend on a number of experimental factors, the main ones being the crystal mosaic spread u, the source size a and the wavelength distribution 2. Earlier theoretical analyses did not provide a procedure for establishing realistic estimates of these in an actual experimental set-up. Therefore, scan range and detector aperture size have been determined, not from the physical features of the experiment, but by ad hoc decisions, sometimes not far removed from 'guesstimates'. By considering the (L\w, L\20) intensity distribution (Mathieson 1982) and by the selection of appropriate scan modes, it is possible to obtain experimental estimates of suitable outer limits of u, a and 2, namely a/u auand bA , and hence to determine appropriate values of the scan range and aperture size for the reflections to be examined. From a comparison of an example using this method with previously published parameters, it is concluded that, in the majority of past cases of conventional analyses, the scan and aperture parameters chosen led (a) to a progressively decreasing L\A bandwidth as 0 ~ 90° and hence to erroneous determinations of temperature factors -a recognized defect-and (b) to a largely unrecognized defect, namely an overestimation of integrated intensity as 0 ~ 0°, corresponding to a significant source of systematic error which could conceal the true extent of extinction.