Multiphase transformation and Ostwald’s rule of stages during crystallization of a metal phosphate
Chung, S.-Y., Kim, Y.-M., Kim, J.-G. & Kim, Y.-J., Nature Phys. 5, 68-73 (2009).
nucleation, Ostwald's rule of stages, olivine
In the traditional model of crystallization, atoms or molecules nucleate into clusters, and the clusters rapidly grow after reaching a critical radius. However, the system does not necessarily transform directly into the most energetically favorable state. That is, there might be intermediate stable states as a system transforms into its ultimate crystalline state. This was predicted by Ostwald in 1897, and is thus called Ostwald's rule of stages.
This paper shows the first atom-scale evidence of Ostwald's rule of stages in an inorganic compound. This has been difficult to do, because of the short time-scales and high temperatures at which crystallization occurs. Chung et al. observed the crystallization of LiFePO4 at 450o C over the course of 3-4 minutes, using in-situ high-resolution electron microscopy (HREM). They were able to confirm the existence of 3 metastable states by studying the diffraction patterns of the structures. The last transfomation yielded the known stable olivine crystalline structure of LiFePO4.