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Long cell axis

The benefit of having the long cell axis close to the rotation axis


For really large unit cells it is often critical that you place the largest cell dimension along the rotation axis to avoid the situation of having the long axis parallel to the direct beam - the worst case for causing overlaps. 

Dauter. Data-collection strategies. Acta Crystallogr D (1999) vol. 55 (Pt 10) pp. 1703-17

It is very diffcult to collect data from crystals which have one very large unit-cell dimension if the latter lies along the X-ray beam. Particularly if the crystal is mosaic, it may be impossible to avoid reflection overlap. It is much better if the longest axis is aligned close to the spindle axis of crystal rotation, because it will then never lie parallel to the beam. 

Unfortunately, plate-like crystals often have the very long cell edge perpendicular to the flat face. It is difficult to mount such crystals across the spindle axis. A goniostat with geometry may be used to reorient the crystal or the mounting loop can be bent to accommodate a flat crystal in a skewed orientation.