How Ions and Molecules Organize to Form Crystals

Humans have always been fascinated by natural crystals, with their wonderfully perfect shapes, colors, and sizes. The inspiration drawn from their delicate morphologies and sometimes incredible sizes has motivated an enthusiastic pursuit of knowledge to understand the formation of these natural wonders. A promising picture is emerging that is painted by brushes from two schools of thought. One treats crystallization as the successive attachment of individual ions or molecules, and the other as an aggregation of nanosized clusters, either in an ordered fashion or in an initially random arrangement followed by self-reorganization into a crystalline structure. The earlier model, the classical theory, is derived from equilibrium thermodynamics and the atomic structures of crystal surfaces. In contrast, the new, nonclassical model is observation-based, thanks to the advent of highresolution imaging techniques. Together, they represent our current state of knowledge as we work towards unraveling the secrets of crystallization.