Does Impurity-Induced Step-Bunching Invalidate Key Assumptions of the Cabrera−Vermilyea Model?

We show that the growth recovery mechanism of the {110} faces on crystals of the pharmaceutical paracetamol in the presence of its intrinsic impurity acetanilide occurs in the same way as in the growth of inorganic KH 2 PO 4 (KDP) crystals with Fe(III) and Al(III) impurities, by initial movement of macrosteps while elementary steps remain pinned. This suggests that the mechanism of recovery by activation of elementary step motion assumed by Cabrera and Vermilyea (C−V) is not applicable to a diverse set of common and technologically important crystal systems. Recognizing that impurity-driven macrostep formation depends on an imbalance in the concentration ahead and behind the step, we propose a general condition that must be met for a crystal-impurity system to behave according to the C−V predictions.