Ab initio Study of Al(III) Adsorption on Stepped 100 Surfaces of KDP Crystals

Crystals of potassium dihydrogen phosphate (KH2PO4, KDP) are grown in large scale for use as nonlinear material in laser components. Traces of trivalent metal impurities are often added to the supernatant to achieve habit control during crystal growth, selectively inhibiting the growth of the {100} face. Model systems representing AlPO4-doped KDP {100} stepped surfaces are prepared and studied using ab initio quantum methods. Results of Hartree–Fock partial optimizations are presented, including estimated energies of ion pair binding to the steps. We find that the PO43− ion takes a position not unlike that of a standard phosphate in the crystal lattice, while the aluminum atom is displaced far from a K+ ion position to establish coordinations with the PO43− ion and to bind with another lattice-bound phosphate. Our optimized structures suggest that it is the formation of a fourth coordination of Al(III) to a third phosphate ion from solution, or perhaps from a nearby position in the lattice, that disrupts further deposition, pinning the steps.