Solution-induced reconstructive epitaxial nucleation on pseudoflat surfaces of fractal gel-grown ammonium chloride

Abstract

The morphological manifestation of regular fractal patterns in ammonium chloride and the transition from ordinary to fractal morphology are studied at the crystallographic and molecular level by means of the Hartman-Perdok theory. The theoretically predicted growth forms as a function of growth conditions are in excellent agreement with experimental results for growth out of vapor ({211} and {110}) and out of polar solutions ({211}, {100}, and/or {111}), in the ordinary as well as fractal morphological regimes. The {211} surface is (2 × 1)-reconstructed due to internal or structural energy gain that persists in all growth conditions. The mechanism responsible for the onset of fractal growth in a gel containing water and agarose molecules is solution-induced reconstructive epitaxial nucleation (SIREN) between pseudoflat pyramidal {211} faces. SIREN is equivalent to the structural match between crystallites, anticipated in the macroscopic theory of nucleation and growth kinetics [Liu, X. Y.; Strom, C. S.; J. Chem. Phys. 2000,113. In press.]. It is associated with an increase in the effective surface supersaturation, brought about by the surface poisoning of the agarose inhibitors. It is characterized by a critical length parameter of the order of or lower than crystallite dimension, and a critical match parameter m → 1, developed in that work. © 2000 American Chemical Society.