A combined ab initio and atomistic simulation study of the surface and interfacial structures and energies of hydrated scheelite:: introducing a CaWO4 potential model

Density functional theory (DFT) calculations of the calcium tungstate material scheelite CaWO4 have shown that water introduced into the bulk material remains undissociated and leads to swelling and layering of the structure, a behaviour which may resemble silicate clays more than three-dimensional poly-anionic materials, but which results in a structure that is even more similar to a rare hydrous calcium carbonate phase-a finding which suggests the existence of semi-crystalline hydrous pre-cursor phases to the dehydrated scheelite material. An interatomic potential model was derived for CaWO4 which adequately reproduces structural and physical properties of the material and is in good agreement with the DFT calculations in respect of the structure and energy of hydration (DFT: 85 kJ mol(-1), atomistic: 105 kJmol(-1)). Atomistic simulations of a range of scheelite surfaces confirm the dominance of the experimental {101} and {001} cleavage planes in the morphology of the dry crystal and the presence of the experimentally found {103} and {101} surfaces in the hydrated morphology. Hydration of the surfaces shows non-Langmuir behaviour, where the interactions between surface calciums and oxygen atoms of the water molecules outweigh hydrogen-bonding to the surface oxygen atoms or intermolecularly within the water layer. The hydration energies indicate physisorption of water, ranging from 22 kJ mol(-1) on the {001} surface to 78 kJ mol(-1) on the more reactive {103} surface. (C) 2003 Elsevier Science B.V. All rights reserved.