Probing the Ba 5d states in BaTiO3 and BaSO4: A resonant x-ray emission study at the Ba-L3 edge

We have directly probed the Ba 5d states in the ferroelectric barium titanate BaTiO₃ using two bulk-sensitive spectroscopic probes, resonant x-ray emission spectroscopy (RXES) and x-ray absorption spectroscopy in the partial fluorescence mode (PFY-XAS) at the Ba-L₃ edge. The results are compared with those of the non-ferroelectric barium sulfate BaSO₄. While the RXES spectra point to a localized character for the Ba 5d states in both compounds, the main peak of the PFY-XAS spectrum, corresponding to the dipolar transitions from 2p to 5d, is found to be significantly broader for BaTiO₃ than for BaSO₄. On the basis of band structure calculations, this broadening is ascribed to strong hybridization between the unoccupied Ba 5d and O 2p states in the ferroelectric. This suggests that the hybridization between the conduction states of the Ba²⁺ and O^2? ions, and not only Ti⁴⁺ and O^2?, plays a central role in determining the electronic structure of BaTiO₃, and is therefore likely to be indirectly correlated with the occurrence of ferroelectricity in this material.