Diffusion mechanisms of native point defects in rutile TiO2:: Ab initio total-energy calculations

The structural energetics and diffusion mechanisms of the two most important point defects in rutile TiO2, the oxygen vacancy (V-O) and the titanium interstitial (Ti-I), are examined using the ab initio pseudopotential total-energy method. The two defects are found to be somewhat competitive in energy, with V-O being more favorable in a larger range of the stoichiometry. The Ti-I, on the other hand, is shown to be the major diffusive species, since its low migration barrier is significantly smaller (by similar to 1 eV) compared to that of V-O. The diffusion mechanisms of Ti-I parallel and perpendicular to the crystal c axis are found to be different with a surprisingly larger barrier along the more open [001] direction, which was originally thought to be the easier channel for Ti self-diffusion. These theoretical findings are in excellent quantitative agreement with existing experimental data.