DIELECTRIC-CONSTANTS AND BORN EFFECTIVE CHARGES OF TIO2 RUTILE

The high-frequency dielectric-constant tensor and the Born effective charge of TiO2 rutile are calculated using variational density-functional perturbation theory. The calculated structural properties and high-frequency dielectric constant show agreement with experiments at the level of a few percent. The Born effective charges of TiO2 rutile are found to be much larger than the charges corresponding to Ti4+ or O2- ions. We compare these results with those obtained for SiO2 stishovite, for which the value of the Born effective charges is closer to an ionic picture.