Surface Atomic Structure of KDP Crystals in Aqueous Solution: An Explanation of the Growth Shape

With this study on KDP, we present an interface atomic structure determination of a crystal in contact with its growth solution. Using x-ray diffraction at a third-generation synchrotron radiation source, the structure of both the ${101}$ and ${100}$ faces has been determined. We found that the ${101}$ faces are terminated by a layer of ${\mathrm{K}}^{+}$ ions and not by ${\mathrm{H}}_{2}{\mathrm{PO}}_{4}^{\phantom{\rule{0ex}{0ex}}\ensuremath{-}}$ groups, resolving a long-standing issue that could not be predicted by theory. This result leads to an atomic-scale explanation of the influence of metal impurities on the macroscopic growth morphology.