The growth spiral morphology on {100} KDP related to impurity effects and step kinetics

The growth spiral shape of the {100} KH₂PO₄ surfaces has been a classical example to elucidate the importance of surface diffusion in the crystal growth process. We will show that this hypothesis is undermined by the pointed growth spiral shape actually observed. On the basis of a recent PBC analysis we propose a step growth mechanism in which direct integration of the growth unit at the kink site plays an important role. In addition, by in-situ microscopy a sharp transition is observed to a circular growth spiral shape upon lowering the supersaturation. This transition is interpreted as an effect of impurity adsorption. The equilibrium chemical composition of the mother liquid itself, instead of M³⁺ ions or other foreign species, is deduced to be responsible for this effect.