Validity of Hartman-Perdok PBC theory in prediction of crystal morphology from solution and surface X-ray diffraction of potassium dihydrogen phosphate (KDP)

Recent literature shows that the validity and capabilities of the Hartman-P erdok (HP) theory of crystal growth and morphology are in need of clarifica tion. The HP theory can predict uniquely, growth layers and growth habits i n terms of the crystal structure and the growth medium in energy-driven gro wth processes. An interpretation of the experimental results on the surface s of potassium dihydrogen phosphate (KDP) (de Vries et al., Phys. Rev. Lett . 80 (1998) 2229), is open to fundamental challenge on the following claims . Prediction of which of the possible alternative surface terminations will control crystal growth in solution is supposed to be impossible, because t he HP computation can deliver many theoretical growth layers. The theory is considered unable to account for the influence of the solution on the inte rface. The distinction of K+ versus H2PO4- pyramidal surface termination is seen as a long-standing issue that could not be resolved by theory. In ion ic compounds like KDP, the atomic structure at the outermost boundary is be lieved to control the growth mechanism and morphology of the crystal, while the presence or absence of cation-anion alternation at that boundary is su pposed to determine the role of polarity in step kinetics. The observed pyr amidal surface cell of KDP is considered to generate a strongly polarized g rowth front. The present reassessment of the experimental results of the ab ove work substantiates the following counterconclusions. The wide variety o f computed growth layers is often diminished by conditions of space group s ymmetry; rather than being a theoretical artefact, it has physical signific ance, since in practice different theoretical growth layers parallel to (hk l) are activated by different growth environments such as vapour, melt or s olutions. Surface electrostatics and the role of polarity on step kinetics cannot be inferred solely from the outermost ion array while ignoring the r est of the growth layer, including the step height. Cation-anion alternatio n at the surface termination is neither a necessary nor a sufficient condit ion for establishing surface polarity (cf Strom, J. Phys. Chem. B 103 (1999 ) 11 339), and moreover it does not necessarily imply alternation of positi ve and negative atomic charges on that plane. Consequently, and consistentl y with the symmetry point located in the middle of the experimentally deter mined d(101) layer cell of KDP, the observed pyramidal surface is unpolariz ed. The occurrence of the K+-terminated, as opposed to the H2PO4--terminate d, pyramidal surface of KDP grown out of an aqueous solution, as well as th e role of ion impurities on the prismatic surface, have been anticipated on firm grounds in the past. Past HP predictions of solution-induced effects on other structures are outlined. (C) 2001 Elsevier Science Ltd. All rights reserved.