Carrier-induced phonon stiffening and localization in rutile TiO2

Crystalline transition metal oxides such as TiO$_2$ possess attractive properties for many practical applications. Despite several decades of researching rutile TiO$_2$ carrier-ion interactions that account for many important properties of this material, such as carrier mobility, effective mass and photo-catalytic activity, are still the subject of controversy. Here we study carrier properties in rutile TiO$_2$ both experimentally, using Raman spectroscopy of photo-excited samples, and computationally, with the real-time time-dependent density functional theory (RT-TDDFT) quantum-classical method, which employs mean-field classical (Ehrenfest) dynamics to couple the electronic and ionic subsystems. From simulation we find that small polaron formation in rutile titania is a strongly non-adiabatic process with the characteristic time scale of about 55 fs. In both experiment and theory we observe an unexpected stiffening of the $A_{1g}$ phonon mode under UV illumination. We computationally analyze the polaron structure and explain the observed effect. The resulting form of the potential with respect to oxygen atoms and $t_{2g}$-orbitals of central Ti atom offers a possible explanation for an anomalous temperature-dependence of the Hall mobility in rutile titania.