(23d) Engineering Crystal Shape and Purity: Where Next?

It is well known that small amounts of foreign molecules (additives or impurities) in solution (at the 10-100ppm level) are capable of having dramatic effects on the shape and impurity uptake of solution-grown crystals. Frequently, these foreign molecules are structurally related to the solute because they result from reaction byproducts. We refer to them, collectively, as imposters. The conventional explanation for shape change due to the presence of imposters is in terms of a mechanism called "step pinning" whereby the impurities adsorb on the terraces of a crystal surface and decrease the step velocity. This, in turn, decreases the perpendicular growth rate of the crystal face and thus changes the crystal shape. In a recent paper, we show that this is not expected at such low imposter concentrations in solution (Sizemore and Doherty, Cryst. Growth & Design, 9, 2637, 2009). Instead, we proposed an entirely new mechanism for growth inhibition in which the distances travelled by the steps in the first turn of the growth spiral are increased, thereby decreasing the density of steps across the face and thus decreasing the perpendicular growth rate of the face. In this presentation I will explain the basis of the new model and discuss its implications for both crystal shape change and impurity uptake.