Doping effect of Nb on ionic polarization of SrTiO₃

Abstract

THz dielectric spectra of Nb-doped SrTiO₃ single crystals were measured using the far-infrared spectroscopic ellipsometer to understand the doping effect of Nb on ionic polarization. The frequency of the Slater transverse optical (TO) mode increased with the doping amount of Nb, meaning the decrease in the ionic polarization by Nb doping. From the change in the Slater TO mode frequency, it was estimated that the ionic polarizability of 1% Nb-doped SrTiO₃ was 10.4% lower than that of undoped SrTiO₃. This result suggested that the intrinsic permittivity of Nb-doped SrTiO₃ is lower than that of that of undoped SrTiO₃. First principles calculations revealed that the Nb 4d-orbitals contributes to the covalency between B-site cations and O ions, however, the covalency is suppressed due to the electron carrier doping. The doped electrons causes the decrease in the long-range Coulomb force and the increase in short-range elastic force, and therefore the ionic polarizability is decreased by Nb doping.