title: Alternating metastable/stable pattern in the mercuric iodide crystal formation outside the Ostwald Rule of Stages.
abstract: We report a reaction-diffusion system in which two initially separated electrolytes, mercuric chloride (outer) and potassium iodide (inner), interact in a solid hydrogel media to produce a propagating front of mercuric iodide precipitate. The precipitation process is accompanied by a polymorphic transformation of the kinetically favored (unstable) orange, (metastable) yellow, and (thermodynamically stable) red polymorphs of HgI2. The sequence of crystal transformation is confirmed to agree with the Ostwald Rule of Stages. However, a region is found of initial inner iodide concentration, where a stationary pattern of alternating metastable/stable crystals is formed. A theoretical model based on reaction diffusion coupled to a special nucleation and growth mechanism is proposed. Its numerical solution is shown to reproduce the experimental results.
authors.affiliation: Department of Chemistry, American University of Beirut , P.O. Box 11-0236, Beirut 1107 2020, Lebanon.
authors.forename: Mahmoud M
authors.initials: MM
authors.lastname: Ayass
authors.forename: Andrew
authors.initials: A
authors.lastname: Abi Mansour
authors.forename: Mazen
authors.initials: M
authors.lastname: Al-Ghoul