Transition Between Periodic Precipitation and Tree-like Crystal Aggregates

Abstract

Transitions between periodic patterns and tree-like patterns in lead iodide precipitation systems were found with varying agar concentration in diffusion media. At high agar concentrations, periodic patterns (Liesegang band) of lead iodide precipitates form whereas at low agar concentrations, tree-like crystal aggregates similar to diffusionlimited aggregation (DLA) develop in the entire system. At an intermediate agar concentration, Liesegang band forms near the contact surface, while tree-like aggregates appear suddenly at some distance from the contact. The influence of agar concentration and lead-iodide concentration ratio was represented as the morphological phase diagram. Together with examinations of diffusion coefficient, nucleation threshold, internal structure of agar media and crystal number densities with varying agar concentration, it is concluded that the effect of agar concentration on nucleation kinetics, rather than on diffusion coefficient, controls the morphological transition.