Theoretical Study of Atomic Fluorine Diffusion through Bulk TiO2 Polymorphs

Diffusion of F through the anatase, brookite, and rutile polymorphs has been studied by periodic density-functional-theory-based calculations and large supercells. Different crystallographic directions for F diffusion have been explicitly considered. F diffusion has been found to be kinetically favored in the case of anatase and rutile, although in the latter the process is thermodynamically disfavored. Diffusion along each direction involves channels between the TiO2 octahedral units in each crystal with different size. However, the analysis of the structures near the minimum and maximum energy values along each diffusion pathway before and after substrate relaxation indicates that considering the size of the channels does not allow one to make accurate predictions because the main effect concerns the different degree of flexibility of each polymorph, which may facilitate or hinder the formation of interstitial complexes.