LATTICE-DYNAMICS AND DIELECTRIC-PROPERTIES OF INCIPIENT FERROELECTRICTIO2 RUTILE

The phonon frequencies at the Gamma point, the Born effective charges, and the dielectric permittivity tensors of TiO2 rutile are calculated using the variational density-functional perturbation theory. The cal culated phonon frequencies agree with experiment within a few percent. We analyse the corresponding theoretical eigenvectors as well as the interaction of the vibration modes with the electric field. The Born e ffective charges of TiO2 rutile are much larger than the nominal charg es of Ti4+ and O2- ions, as well as those of SiO2 stishovite despite t he structural similarity. A giant LO-TO splitting is observed for E(mu ) and A(2 mu) modes. The O ions in rutile as well as in stishovite are found to have a counterintuitive reversed electronic polarization nea r the nuclei. The calculated large, anisotropic static dielectric perm ittivity tensors as well as electronic dielectric permittivity tensors also compare favorably with experiments.