SMALL CRYSTALS ARE OFTEN POOR CRYSTALS - WHY? Marjorie M Harding, Robert J Rule, Deapartment of Chemistry, University of Liverpool PO Box 147, Liverpool L69 3BX, UK

Diffraction patterns recorded with synchrotron radiation have given us some clues. We have recorded synchrotron radiation diffraction data for structure determination for a substantial number of very small single crystals (< 0.02 mm for example). In most cases the use of synchrotron radiation was a 'last resort' because it had not proved possible to grow crystals of good size and quality suitable for use with conventional Xray generators and diffractometers.

A large proportion of small crystals are either very thin plates or very fine needles; this is probably associated with poor growth in one or more directions in the structure. Three kinds of observation support this view:

a) For many of the small crystals studied because they would not grow bigger, the mosaic spread is unusually large (it can be quite accurately estimated in synchrotron radiation experiments because of the low beam divergence).

b) Following structure determination it is often possible to identify structural features which could explain the potential for faulted growth. Examples will be given.

c) The geometry of the Laue method for recording single crystal diffraction patterns is particularly sensitive to mosaic spread. By this method we have shown, for two compounds which form very fine needle crystals, that the mosaic spread is anisotropic, and is larger in the direction perpendicular to the needle axis than parallel to it. One structure is an aluminophosphate, with pores parallel to the needle axis; the other is organic, but also has large channels parallel to the needle axis.

So although synchrotron radiation has allowed structural information to be derived for a number of very small crystals, many of them were faulted. This prevented us from obtaining geometrical data of the highest quality, and probably also prevented them from growing larger.