Dealing in Small Volumes

Although pressure-induced solid-state transformations are important in both geology and materials science, they are difficult to understand at a fundamental level because their kinetics can be highly inhomogeneous, and the role of defects can make transition cycles irreproducible. Jacobs et al. (p. 1803) tackled this problem by shrinking the sample size to increase homogeneity and eliminate defects. They studied the transition between in CdSe nanoparticles between the low-pressure tetrahedrally bonded phase and the high-pressure, six-coordinate phase. The activation volumes for the forward and reverse transitions are opposite in sign, which indicates the intermediate has a volume in between that of the fourand six-coordinate phases (rather than greater than both). This finding suggests that directionally dependent nucleation occurs that would be consistent with shearing motion along the crystal planes.