Size effect study by bulk crystals: Optical and DC dielectric response of oxygen vacant BaTiO₃

We have studied the wide-range dielectric spectra of BaTiO _3-δ single crystals and traced the evolution of the electronic structure of BaTiO₃ with a very low oxygen vacancy concentration δ An extremely small δ can destroy the band edge and the Urbachian temperature dependence, and a so-called mid-infrared (ir) absorption band which shows no hump like absorption spectrum. At a slightly larger δ, an absorption spectrum has a hump like shape and exhibits temperature dependence distinctly different below and above Curie temperature. At this δ, the permittivity is enhanced by five times and exhibits a large frequency dispersion. The observations suggest a possible coupling of oxygen vacancy with a mesoscopic spontaneous polarization and elucidate the origin of mid-ir absorption generally observed in doped oxides. The implications for relaxor and the size effect are discussed.