Hydrogen motion in rutile TiO2

Abstract

Uniaxial-stress experiments have been performed for the 3287- and 2445-cm-1 local vibrational modes assigned to the positive charge state of interstitial hydrogen ({{\rm{H}}}-{i}^{+}) (H i +) and deuterium ({{\rm{D}}}-{i}^{+}) (D i +), respectively, occurring in mono-crystalline rutile TiO2. The onset of the defect alignment under the stress applied perpendicular to the [001] axis is detected at 165 K (185 K), which corresponds to the activation energy of 0.53 eV (0.58 eV) for interstitial hydrogen (deuterium). Based on these findings the diffusion constants of {{\rm{H}}}-{i}^{+} H i + and {{\rm{D}}}-{i}^{+} D i + along the [001] axis of TiO2 are determined. The experimental data are complemented by density-functional theory calculations and compared with the earlier results on the diffusion of {{\rm{H}}}-{i}^{+} H i + / {{\rm{D}}}-{i}^{+} D i + at elevated temperatures up to 700 °C. It is found that the activation energy value deduced from our low-temperature stress measurements yields a very good agreement with the high-temperature data, covering a dynamic range of 12 orders of magnitude.