Design of a low band gap oxide ferroelectric: Bi₆Ti ₄O₁₇

A strategy for obtaining low band gap oxide ferroelectrics based on charge imbalance is described and illustrated by first-principles studies of the hypothetical compound Bi₆Ti₄O₁₇, which is an alternate stacking of the ferroelectric Bi₄Ti₃O ₁₂. We find that this compound is ferroelectric, similar to Bi ₄Ti₃O₁₂ although with a reduced polarization. Importantly, calculations of the electronic structure with the recently developed functional of Tran and Blaha yield a much reduced band gap of 1.83 eV for this material compared to Bi₄Ti₃O₁₂. Therefore, Bi₆Ti₄O₁₇ is predicted to be a low band gap ferroelectric material.