Probing the Ba 5d states in BaTiO 3 and BaSO 4 : A resonant x-ray emission study at the Ba-L 3 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 O2− ions, and not only Ti4+ and O2−, 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.