THERMAL-DIFFUSE-SCATTERING CORRECTIONS FOR SINGLE-CRYSTAL DIFFRACTION DATA FROM AREA DETECTORS. G.J. McIntyre, Institut Laue-Langevin, B.P. 156, 38042 Grenoble Cedex 9, France.

X-ray and neutron single-crystal diffraction studies on the same compound consistently give comparable coordinates, but frequently quite different displacement parameters. Attributed mainly to uncorrected thermal-diffuse-scattering (TDS) contributions, these differences can be considerably reduced by using an area detector in the neutron experiment to give an integration envelope comparable in size in reciprocal space to the integration envelope of the X-ray experiment.

But there is a more important benefit afforded by area detectors! If the resolution of the diffractometer is assumed to be infinitely small the amount of one-phonon TDS included in the scan through a Bragg reflection is directly proportional to the radius of the peak integration volume, the amount of two-phonon TDS to the square of the radius, and the amount of incoherent (flat) background to the cube of the radius. These differences in the dependence on the size of the integration volume can be exploited to correct for TDS and to estimate the elastic constants empirically, provided each reflection is sampled in three dimensions, as in scans made with an area detector.

The TDS corrections for X-ray or neutron intensities derived by summation of counts in three dimensions will be discussed in detail. The precision in the empirical method as described above is poor for weak reflections, but, because of the slow variation of TDS with the scattering vector, the corrections for these reflections can be estimated from those of nearby strong reflections. Another advantage offered by area detectors is optimal delineation of peak and background to minimise the estimated error in the background-corrected integrated intensity. For weak reflections this might imply integration within an envelope smaller than the instrumental resolution volume. Exact correction can even be made in this circumstance, within the reasonable assumption that the integration envelope and the resolution volume have the same shape. In a similar fashion allowance can also be made for the TDS contributions from neighbouring reflections.