Structure and Stability of TiO₂-B Surfaces: A Density Functional Study

We investigate the structure and energetics of low-index surfaces of the TiO₂-B polymorph by means of periodic density functional theory calculations within the generalized gradient approximation. The bulk structure contains two nonequivalent Ti ions, one of them exhibiting an octahedral coordination, while the other is square-pyramidal. When exposed at the surface, these two types of ions display different relaxation schemes, which ultimately tend to make them more similar. On the basis of the computed surface energies and of the Wulff construction, we predict for TiO₂-B a pseudohexagonal prismatic equilibrium shape and an average surface energy practically identical to that of TiO₂-anatase.