The crystal must be smaller than size of the incident beam at the
crystal. For most diffractometers with fine-focus X-ray tubes and
monochromated radiation, this size is unlikely to be much greater
than 0.5 mm. Note that increasing the collimator diameter will not
necessarily increase the size of the incident beam at the crystal.
If the crystal used in the data collection is larger than the
incident beam, then unpredictable errors in intensities will occur
due to the varying amounts of the crystal in the beam for different
reflection positions. If you obtain an alert about crystal size,
then data should be collected with a smaller crystal. For the
above reasons, it is recommended that crystals be generally smaller
than 0.5 mm along any edge.
If you used neutrons and obtained an alert about the crystal size, then you have forgotten to set _diffrn_radiation_type 'neutron'.
Copyright © International Union of Crystallography