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

The morphological manifestation of regular fractal patterns in ammonium chl oride and the transition from ordinary to fractal morphology are studied at the crystallographic and molecular level by means of the Hartman-Perdok th eory. The theoretically predicted growth forms as a function of growth cond itions art: in excellent agreement with experimental results for growth out of vapor ((211) and (110)) and out of polar solutions ((211), (100), and/o r (111)), in the ordinary as well as fractal morphological regimes. The (21 1) surface is (2 x 1)-reconstructed due to internal or structural energy ga in that persists in all growth conditions. The mechanism responsible for th e onset of fractal growth in a gel containing water and agarose molecules i s solution-induced reconstructive epitaxial nucleation (SIREN) between pseu doflat pyramidal (211) faces. SIREN is equivalent to the structural match b etween crystallites, anticipated in the macroscopic theory of nucleation an d growth kinetics [Liu, X. Y.; Strom, C. S.: J, Chem. Phys. 2000, 113. In p ress.]. It is associated with an increase in the effective surface supersat uration, brought about by the surface poisoning of the agarose inhibitors. It is characterized by a critical length parameter (I) of the order of or l ower than crystallite dimension, and a critical match parameter m --> 1, de veloped in that work.