Orthorhombic distortion on Li intercalation in anatase

The mechanism of the tetragonal-to-orthorhombic transformation upon Li-intercalation into anatase structured titania has been studied using first principle calculations. The primary mechanism for the formation of the orthorhombic phase is found to be the accommodation of donated charge in localized $\mathrm{Ti}{-d}_{\mathrm{yz}}$ orbitals leading to a cooperative Jahn-Teller-like distortion of the lattice. This model is examined further by considering electron addition states in pure anatase and the analogous structures of ${\mathrm{H}}_{0.5}{\mathrm{TiO}}_2$ and ${\mathrm{Na}}_{0.5}{\mathrm{TiO}}_2.$ It is shown that the rigid band model is not valid and population of the degenerate $\mathrm{Ti}{-d}_{xy,yz}$ orbitals occurs beyond a critical concentration due to the repulsive interaction of the localized electrons. It is shown that the stability of the ${\mathrm{Li}}_{0.5}{\mathrm{TiO}}_2$ structure is related to the similarity of the ionic radius of ${\mathrm{Li}}^{+}$ and ${\mathrm{Ti}}^{+3\mathrm{}}$ ions. Optimal configurations of ${\mathrm{H}}_{0.5}{\mathrm{TiO}}_2$ and ${\mathrm{Na}}_{0.5}{\mathrm{TiO}}_2$ are also predicted.