Activation volumes for solid-solid transformations in nanocrystals

The transition between four- and six-coordinate structures in CdSe nanocrys tals displays simple transition kinetics as compared with the extended soli d, and we determined activation volumes from the pressure dependence of the relaxation times. Our measurements indicate that the transformation takes place by a nucleation mechanism and place strong constraints on the type of microscopic motions that lead to the transformation. The type of analysis presented here is difficult for extended solids, which transform by complic ated kinetics and involve ill-defined domain volumes. Solids patterned on t he nanoscale may prove to be powerful models for the general study of struc tural transitions in small systems, as welt as in extended solids.