Observation of two-dimensional nucleation on the {0 1 0} face of potassium hydrogen phthalate (KAP) crystals using ex situ atomic force microscopy

The surface topology of microcrystals of potassium hydrogen phthalate (KAP) grown at a relative supersaturation of approximately 26% has been investigated using ex situ atomic force microscopy (AFM). Two distinctly different plate-like habits were observed. The thicker of the two habits resulted from growth by a spiral mechanism on the {0 1 0} faces while a thinner, less common, plate-like morphology occurred due to growth by two-dimensional nucleation on this face. The nuclei, being both asymmetric and partially polygonised, are essentially identical in shape to that of the spirals observed on the thicker crystals. They range in size from about 1200 to 400 nm with the smallest of the nuclei being substantially larger than the estimated radius of the critical two-dimensional nucleus at this supersaturation. This is attributed to continued growth of the nuclei, during separation from the growth solution. The nuclei are distributed non uniformly across the surface of the crystals, being greatest in number at the edges and corners of the, face. This {0 1 0} distribution reflects the expected variation in surface supersaturation across the crystal face. (C) 1998 Elsevier Science B.V. All rights reserved.