  Band gap engineering in SrTiO${}_{3}$ and related titanate perovskites has
long been explored due to the intriguing properties of the materials for
photocatalysis and photovoltaic applications. A popular approach in the
materials chemistry community is to substitutionally dope aliovalent transition
metal ions onto the B site in the lattice to alter the valence band. However,
in such a scheme there is limited control over the dopant valence, and
compensating defects often form. Here we demonstrate a novel technique to
controllably synthesize Fe$^{2+}$- and Fe$^{3+}$-doped SrTiO${}_{3}$ thin films
without formation of compensating defects by co-doping with La$^{3+}$ ions on
the A site. We stabilize Fe$^{2+}$-doped films by doping with two La ions for
every Fe dopant, and find that the Fe ions exhibit a low-spin electronic
configuration, producing optical transitions in the near infrared regime and
degenerate doping. The novel electronic states observed here offer a new avenue
for band gap engineering in perovskites for photocatalytic and photovoltaic
applications.

Materials Science