Structure and Stability of α-AlF3 Surfaces

An understanding of the phase stability of AlF3 surfaces as a function of their environment is an important prerequisite in the development of, and an ability to control, their catalytic properties. In this study, all electron hybrid-exchange density functional theory is used to calculate the structure and corresponding energies of several α-AlF3 surfaces. It is shown that the surfaces expose under-coordinated Al ions that are potential Lewis acid sites. The binding energy of NH3 to these sites is calculated and used to quantify their relative acidities. The Lewis acid sites are significantly weaker than the strongest sites predicted to occur on β-AlF3 surfaces. The equilibrium morphology of α-AlF3 crystallites is predicted from the construction of an approximate Wulff plot. The stabilities of two representative terminations of α-AlF3, as a function of HF and H2O chemical potentials are computed using ab initio thermodynamics. The geometries of their stable surfaces are found to be strongly dependent on t...