The origin of low bandgap and ferroelectricity of a co-doped BaTiO3

We recently demonstrated a new low bandgap ferroelectric material based on BaTiO3. The co-doping of BaTiO3 exhibited robust spontaneous electrical polarization with bandgap values suitable for visible light absorption for application in optoelectronic devices. In this study, the valence and conduction bands are investigated with a combination of x-ray spectroscopies and DFT calculations. The local electronic structure and coordination of BaTi1-x(Mn1/2Nb1/2)xO3 is studied by means of x-ray absorption at the Ti K, Mn K, and O K edges. The spectroscopic evidence suggests only small distortions to the parent tetragonal ferroelectric system, reducing of the bandgap through compositional doping without compromising ferroelectricity to a large extent. The Ti K pre-edge features in particular, which are sensitive to site coordination and an indication of Ti off-centering within the Ti-O6 octahedra, shows modest changes with doping and strongly corroborates with our measured polarization values. Resonant photoemission spectroscopy results reveal newly created Mn d bands that hybridize with O 2p as well as additional valence band edge states with predominantly Mn d character. Through various x-ray spectroscopic techniques, we reveal the electronic structure that allows the compound to retain its ferroelectricity while reducing the bandgap