Ewald ionic maps gauging the effect of solvent interaction on crystal morphology illustrated by surface X-ray diffraction of potassium dihydrogen phosphate

Ewald maps provide an exact assessment of the fine-scale electrostatic beha vior of the combined bulk-cell and surface-cell trial solutions used in int erpreting X-ray diffraction data on ionic surfaces. Thus, insight can be ga ined in the effect of the surface electric-field distribution and surface p olarity on crystal morphology. A spatial distribution of the solvent- or im purity-accessible surface locations is determined as a function of the van der Waals spheres of the solid and fluid species. The electrostatic potenti al and electric field vector are computed on the resulting undulated input surface, by-means of an analytical formulation of the Ewald method adapted to laminas, Equipotential and equifield contours enable the identification of possible adsorption sites of cations on local potential minima, anions o n local:local potential maxima, and neutral polar particles (e.g., water) o n local field maxima. Experimentally observed surface reconstruction can be accounted for by distinguishing between a "surface cell" generating the to p (hkl) layer adjacent to the liquid and a "bulk cell" generating all subse quent layers. Exactness and model independence avoid fundamental inconsiste ncies inherent in approximate and intuitive approaches encountered in recen t literature. The electric field distribution on the crystal surface determ ines the effect of a polar liquid on the growth form. General applicability to structures with a dominant ionic character is ensured. An application t o potassium dihydrogen phosphate, K+H2PO4- (KDP), is presented.