Multiphase transformation and Ostwald's rule of stages during crystallization of a metal phosphate

Although the classical picture of crystallization depicts a simple and immediate transformation from an amorphous to a crystalline phase, it has been argued that, in selected systems, intermediate metastable phases exist before a stable state is finally reached. However, most experimental observations have been limited to colloids and proteins, for which the crystallization kinetics are fairly slow and the size is comparatively large. Here, we demonstrate for the first time in an inorganic compound at an atomic scale that an amorphous phase transforms into a stable crystalline state via intermediate crystalline phases, thus directly proving Ostwald's rule of stages. Through in situ high-resolution electron microscopy in real time at a high temperature, we show the presence of metastable transient phases at an atomic scale during the crystallization of an olivine-type metal phosphate. These results suggest a new description for the kinetic pathway of crystallization in complex inorganic systems.