Structure and Stability of α-AlF₃Surfaces

Abstract

An understanding of the phase stability of AlF 3 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-AlF 3 surfaces. It is shown that the surfaces expose under-coordinated Al ions that are potential Lewis acid sites. The binding energy of NH 3 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-AlF 3 surfaces. The equilibrium morphology of α-AlF 3 crystallites is predicted from the construction of an approximate Wulff plot. The stabilities of two representative terminations of α-AlF 3, as a function of HF and H 2 O chemical potentials are computed using ab initio thermodynamics. The geometries of their stable surfaces are found to be strongly dependent on the environmental conditions.