Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

tion volumes were also shown to be of opposite sign, indicating that themechanism by which the phase transformation takes place involves a structurewhose volume is in between that of the two end phases. Most significantly,the magnitude of the activation volume is small compared to the totalvolume change that is characteristic of the system (f0.2% versus 18%). Theactivation volume is equal to the critical nucleus size responsible for initiatingthe phase transformation—defining the volume change associated with thenucleation event.The small size of the activation volume suggests that the structuralmechanism for transformation cannot be a coherent one involving the entirenanocrystal [53]. Spread out over the entire volume of the nanocrystal, theactivation volume would amount to a volume change smaller than that inducedby thermal vibrations in the lattice. Therefore, a mechanism involvingsome fraction of a nanocrystal was considered. The nucleus was determinednot to be three dimensional, as a sphere the size of the activation volumewould be less than a single unit cell. Also, activation volumes were observed toincrease with increasing particle size (in the direction of increasing pressure).There is no obvious mechanistic reason for a spherically shaped nucleus toincrease in size with an increase in particle size. Further, additional observationshave been made: (1) particle shape changes from cylindrical or ellipticalto slablike upon transformation from the four-coordinate phase to the sixcoordinatephase [51], (2) the stacking-fault density increases following a fullpressure cycle from the four-coordinate through the six-coordinate and backto the four-coordinate structure [51], and (3) the entropic contribution to thefree-energy barrier to transformation increases with increasing size (indicatingthe nucleation event can initiate from multiple sites) [53]. Together, thevarious experimental observations suggest that the mechanism involves adirectionally dependent nucleation process that is not coherent over the wholenanocrystal. The specific proposed mechanism entails shearing of the (001)planes, with precedent found in martensitic phase transitions (Fig. 22) [51,53].Further, the early observation that activation energy increases with size [50]likely results from the increased number of chemical bonds that must bebroken for plane sliding to occur in large nanocrystals, compared to that insmall nanocrystals. Such a mechanistic-level understanding of the phasetransformation processes in nanocrystals is important because nanocrystalbasedstudies, due to their simple kinetics, may ultimately provide a betterunderstanding of the hard-to-study, complex transformations that occur inbulk materials and geologic solids [53].Phase control, much like shape control (Sect. IV), can be achieved innanocrystal systems by operating in kinetic growth regimes. Materials synthesisstrategies have typically relied upon the use of reaction conditions farfrom standard temperature and pressure (STP) to obtain nonmolecularCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.