  Fabricating complex transition metal oxides with a tuneable band gap without
compromising their intriguing physical properties is a longstanding challenge.
Here we examine the layered ferroelectric bismuth titanate and demonstrate
that, by site-specific substitution with the Mott insulator lanthanum
cobaltite, its band gap can be narrowed as much as one electron volt, while
remaining strongly ferroelectric. We find that when a specific site in the host
material is preferentially substituted, a split-off state responsible for the
band gap reduction is created just below the conduction band of bismuth
titanate. This provides a route for controlling the band gap in complex oxides
for use in emerging oxide opto-electronic and energy applications.
