An Edge-Enhancing Crystal Growth Instability Caused by Structure-Dependent Attachment Kinetics

We describe a novel crystal growth instability that enhances the development of thin edges, promoting the formation of plate-like or hollow columnar morphologies. This instability arises when diffusion-limited growth is coupled with structure-depdendent attachment kinetics, specifically when the nucleation barrier on a facet surface decreases substantially as the facet width approaches atomic dimensions. Experimental data are presented confirming the presence of this instability in the growth of ice from water vapor at -15 C. We believe this edge-enhancing effect plays an important role in determining the growth morphologies of atmospheric ice crystals as a function of temperature, a phenomenon that has been essentially unexplained for over 75 years. Our model of structure-dependent attachment kinetics appears to be related to surface melting, and thus may be present in other material systems, whenever crystal growth from the vapor phase occurs near the material melting point.