In Situ Atomic Force Microscopy Investigation of the {100} Face of KH2PO4 in the Presence of Fe(III), Al(III), and Cr(III)

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Abstract
Here we report the effects of dissolved metal complexes of Fe(III), Al(III), and Cr(III) on the step velocities of the [100] face of KH2PO4 (KDP) as observed with atomic force microscopy. The dependence of step velocity on supersaturation (sigma) exhibits a dead zone that scales with adsorbate concentration. The observed dependence varies with the metal complex. From these data, we derive values for the characteristic adsorption time (tau) for the Al(III) and Cr(III) step-pinning adsorbates as being on the order of several hundred microseconds as compared to 10-100 s for the corresponding Fe(III) step-pinning adsorbates. The values of tau are strikingly different than rates of ligand exchange but are associated with the adsorbate-induced morphology of the surface, including elementary steps that bunch into macrosteps and supersteps. The stoichiometry of the adsorbate species is assumed to be M(HxPO4)x, where M = Fe(III), Al(III), or Cr(III). KDP crystals grown in the presence of the dissolved metals were analyzed using laser ablation inductively coupled plasma mass spectroscopy. The data revealed sectoral zoning on the [100] face, with the concentrations of the incorporated adsorbates in the sector with slower moving elementary steps being 1.7-2.0 times greater than those measured on the sector with fast moving elementary steps.