Structure of Habit-Modifying Trivalent Transition Metal Cations (Mn3+, Cr3+) in Nearly Perfect Single Crystals of Potassium Dihydrogen Phosphate As Examined by X-ray Standing Waves, X-ray Absorption Spectroscopy, and Molecular Modeling

The local atomic structure of habit-modifying transition metal cations within the crystal lattice of potassium dihydrogen phosphate (KDP) is investigated using X-ray absorption spectroscopy (XAS) and X-ray standing wave (XSW) spectroscopy, together with molecular modeling. XAS reveals the transition metal cations to be structurally incorporated into the crystal lattice as an ionic complex that is octahedrally coordinated to two phosphate groups and four water molecules. The position of transition metal cation, as determined by XSW, is consistent with its location at an interstitial lattice site, with a coherent position 0.66 with respect to the {200} crystal lattice planes. The structural model suggested that the transition metal complex mimics the surface structure of KDP prismatic {100} face, hence facilitating its adsorption on this face. Charge compensation associated with the impurity incorporation during crystal growth is effected via the hydrated impurity complex displacing one bonding proton that binds two phosphate groups together with two potassium ions within the crystal structure to maintain the system charge balance. The resulting model is consistent with the XAS and XSW results as well as earlier work (Barrett, N.; Lamble, G. M.; Roberts, K. J.; Sherwood, J. N.; Greaves, G. N.; Davey, R. J.; Oldman, R. J.; Jones, D. J. Cryst. Growth 1989, 94, 689), albeit contrasting with our preliminary data on the isomorphous ammonium dihydrogen phosphate in which a substitutional model was proposed (Cunningham, D. A. H.; Hammond, R. B.; Lai, X.; Roberts, K. J. Chem. Mater. 1995, 7 (9), 1690).