Prediction of a Low Band Gap Oxide Ferroelectric*

We report a first principles study of Bi$_6$Ti$_4$O$_{17}$ which is an alternate stacking of ferroelectric Bi$_4$Ti$_3$O$_{12}$ (BiT). We the standard PBE GGA functional for the structure and polarization and a recently developed functional that yields accurate band gaps for the electronic structure. We find that this compound is ferroelectric although with a reduced polarization relative to BiT. Importantly, calculations of the electronic structure yield a band gap of approximately 1.4 eV. Therefore, we predict that this stacking is a low band gap oxide ferroelectric.