Crystal growth of KDP from boiling solutions in the presence of impurities

Abstract

The effectiveness of FeCl3, AlCl3 and CrCl3 impurities on KDP crystals tapering, can be explained by means of their different rates of ligand exchange from the complexes formed by the metal with H2O and HPO2-4 in the solution. The incorporation of the impurity to the crystal takes place through a surface or kink site, by means of the hydrogen bridge interaction between one HPO2-4 group of the metal complex and a phosphate group of the crystal surface. Then, the adsorbed impurity complex blocks the displacement of growth steps producing tapering. The tapering effect, therefore, depends on the rate of ligand exchange between the metal and the complex constituents. The higher the rate of exchange, the quicker the adsorbed impurity will leave the surface. At higher temperatures, the rate of ligand exchange increases and the tapering effectiveness decreases.