GROWTH-RATE DISPERSION IN SMALL CRYSTALS AND ITS RELATION TO MOSAIC SPREAD

High mosaic spread is a common feature of a number of complex materials (such as certain zeolites and proteins) for which the preparation of large single crystals is prohibited. In one previous systematic study, a direct correlation between growth rate and mosaic spread was found for very small crystals of sodium chlorate. This work has attempted to explore the generality of the effect by extending measurements to a wider range of simple materials. Small crystals (5-200 mum) of ammonium dihydrogen phosphate (ADP), sodium chlorate and sucrose, generated by secondary nucleation in aqueous solution, have been grown under constant conditions of supersaturation. A wide dispersion of growth rates was observed for each material using optical microscopy. A number of individual crystals of known growth rate were successfully retrieved from solution and an assessment of the mosaic spread of each crystal was made using synchrotron radiation Laue diffraction at Daresbury Laboratory. All of the crystals produced diffraction patterns comprising small, sharp spots, indicative of low mosaic spread ( < 0.5-degrees). No correlation was found between growth rate and mosaic spread for these relatively simple structures. In addition, individual crystals of sucrose have also been examined immediately after creation in solution and subsequently grown-on at constant supersaturation. The Laue patterns obtained for each crystal, both before and after growth, exhibited low mosaic spread, yet each crystal grew at a different rate. It is suggested that radiation damage can account for the previously published discrepancies, and acceptable radiation doses are calculated for the determination of mosaic spread in these materials.