Reaction-Diffusion Framework: The Mechanism of the Polymorphic Transition of α- to β‑Cobalt Hydroxide

A new and simple method is proposed to explore the mechanism of the intercalation/deintercalation of a variety of anions throughout the formation of α-Co(OH)2 crystals and their polymorphic conversion to β-Co(OH)2. This method is based on the reaction-diffusion of hydroxide ions in a gel matrix containing the cobalt salt. The spatiotemporal evolution of each polymorph and their interaction is revealed by tracking the location of the two sharp interfaces between the two polymorphs (conversion zone) and between the gel and α-Co(OH)2 (formation zone) and by measuring the weight composition of each zone. We thereby find that the dynamics of the transformation reaction are correctly described by the two-dimensional Avrami-Erofe’ev equation at different temperatures. The data suggest that the structural redistribution of the atoms inside the α-Co(OH)2 particles plays the fundamental role in establishing the overall rate of the reaction. On the other hand, we notice that other factors such as the nature of the intercalated anions and the concentration of the polymer matrix alter considerably the final rate of the transition reaction through increasing the stability of the α phase.