The growth morphology of the {100} surface of KDP (archerite) on the molecular scale

The surface morphology of the {100} face growth hillock of potassium dihydrogen phosphate (KDP) is examined through first-principles calculations and using periodic-bonded-chain (PBC) theory. KDP, or archerite ([K, NH4]H2PO4), is used extensively in industry and is an excellent model for more complicated materials that have considerable ionic and hydrogen bonding as part of their structure. Here, we have calculated the gas-phase detachment energies of KH2PO4 growth units adsorbed to steps oriented normal to the [0 10] and [001] PBC directions. The detachment energies of growth units adsorbed to the two different terminations of the {010}-facing step are +4.2 and +4.5 eV, and detachment from the {011}-facing step is +3.8 eV. Detachment from the {010}-facing step is more unfavorable, indicating that the adsorbed species will be less labile and the step will advance more quickly than the {001}-facing step. The relative detachment energies are qualitatively consistent with experimentally observed fast and slow step velocities on the {100} growth hillock. Detachment energies of growth units adsorbed subsequently to the {010}-facing step are similar, but detachment of a second growth unit from the {001}-facing step is much more unfavorable, +5.1 eV. The increase in detachment energy indicates that the high rate of detachment of the initial KH2PO4 unit from the {001}-facing step may limit the advance of the step, whereas the second growth unit detaches much more slowly. To explain this behavior, we propose that the first growth unit adsorbed to the {001}-facing step has a higher lability because of the lack of hydrogen bonds to the step edge. This study provides a qualitative picture of how crystal structure may control growth morphology of KDP and emphasizes the importance of anisotropic hydrogen bonding in the system.