DIELECTRIC-CONSTANTS AND BORN EFFECTIVE CHARGES OF TIO2 RUTILE

The high-frequency dielectric-constant tensor and the Born effective c harge of TiO2 rutile are calculated using variational density-function al perturbation theory. The calculated structural properties and high- frequency dielectric constant show agreement with experiments at the l evel of a few percent. The Born effective charges of TiO2 rutile are f ound to be much larger than the charges corresponding to Ti4+ or O2- i ons. We compare these results with those obtained for SiO2 stishovite, for which the value of the Born effective charges is closer to an ion ic picture.