Substitution-induced midgap states in the mixed oxides RxBa1−xTiO3−δ with R=Y, La, and Nd

Abstract

Changes induced in the electronic structure of ${\mathrm{BaTiO}}_3$ by substitutions of R=Y, La, or Nd for Ba to form the mixed oxides ${\mathit{R}}_{\mathit{x}}$ ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{TiO}}_{3\mathrm{-}\mathrm{\delta }}$ have been investigated using ultraviolet photoelectron spectroscopy. Substitution of formally ${\mathit{R}}^{3+}$ ions for ${\mathrm{Ba}}^{2+}$ leads to the introduction of filled states in the band gap that are shown by resonant-photoemission measurements to have significant Ti 3d character, consistent with a Mott-Hubbard-insulator description for these oxides. Various contributions to the binding energy and width of these states are considered. It is suggested that the dominant factor is electron-electron correlation and this leads to the estimates U∼Δ∼3 eV for this system, where U is the correlation energy for the 3d electrons and Δ is the charge-transfer energy.