Designing a Lower Band Gap Bulk Ferroelectric Material with a Sizable Polarization at Room Temperature

A low band gap ferroelectric material with a sizable polarization at ambient conditions would constitute an ideal photovoltaic material to harvest solar energy because of its efficient polarization-driven charge carrier separation. We achieve this elusive goal by codoping a Jahn–Teller Mn³⁺ and Nb⁵⁺ pair for two Ti⁴⁺ ions in ferroelectric BaTiO₃. Representing a charge-neutral dipole doping, this approach achieves for the first time a bulk ferroelectric oxide with the lowest band gap of 1.66 eV with a sizable polarization of nearly 70% of BaTiO₃. We contrast this with the analogous system with Mn³⁺ replaced by the non-Jahn–Teller Fe³⁺ (3d⁵) ion, which even at a much lower level of doping reduces the polarization to 25% without reducing the band gap significantly, establishing the efficacy of the present strategy.