Composition and structure of the alpha-AlF3(0001) surface.

Strong Lewis acid catalysts are widely used in a variety of industrial processes including Cl/F exchange reactions. Aluminum fluorides (AlF3) have great potential for use in such reactions. Despite the importance of the surface in the catalytic process little is known about the detailed atomic scale structure of AlF3 surfaces. In the current study we employ state of the art surface thermodynamics calculations based on hybrid-exchange density functional theory to predict the composition and structure of the basal plane surface of alpha-AlF3 for the first time. We examine four possible terminations of the alpha-AlF3 (0001) surface and demonstrate that the surface is terminated by a layer containing two fluorine atoms per cell at all realistic fluorine partial pressures. The fluorine ions in the outermost layer of the material reconstruct to mask the Al3+ ion from the external gas phase and consequently we would expect this surface to be inactive as a Lewis acid catalyst in line with experimental observation.