Computational modeling of self-trapped electrons in rutile TiO2.

In conjunction with the constrained density functional theory, a valence-bond representation has been employed to model the migration of anionic polaron in bulk rutile TiO2. It was found that the charge delocalization of a self-trapped electron proceeded predominately along the c crystal axis of rutile, thus exhibiting pronounced directional heterogeneity of polaron migration. As a result, the extrapolated polaron activation energies are 0.026 eV and 0.195 eV along the [001] and [111] lattice vectors, respectively. According to the Holstein theory, the difference on the activation energy makes the polaron drift over 100 times faster along the c crystal axis than on the ab crystal plane at room temperature. The notable anisotropy of the anionic polaron was also reflected through the electron paramagnetic resonance (EPR) g-matrix, whose principal component along [001] is substantially smaller than that along [110] or [11̅0]. Finally, the extent of polaron charge was probed by our calculated isotropic hyperfine coupling constants on two groups of crystallographically inequivalent (17)O atoms, which manifest distinct strengths of spin-orbit interaction with the unpaired electron.