For large unit cells, one must take particular care with the X-ray beam and the orientation of the crystal. The latter has already been mentioned in previous response. For the beam, some things to do are: 1. Make crystal smaller. 2. Make beam smaller (use a smaller collimator size). 3. Reduce beam divergence (change the divergence setting of your optic). 4. Focus beam on the detector and not on the crystal.
Also be aware that by definition large unit cells have small mosaicity. If they didn't, one would not even be collecting data because the spots would be all smeared together due to overlap. No amount of fine-slicing will help resolve spots that are overlapped in the rotation direction. Jim >... >I've collected a Pt soak data set on our home source with a 0.5˚ oscillation >angle, but the anomalous signal drops off after about 8Å. I am wondering if >this is a problem due to overlaps at higher resolution. .... Should I be concerned with this? The crystal mosaicity from XDS is 0.25, so fairly low. What can I do about this, should I try smaller oscillation angles? Thanks, Jason.
