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.