Pressure-induced metal-insulator transition in degenerately doped ferroelectrics*

We perform first-principles calculations to study oxygen vacancies in two important types of ferroelectric oxides (LiNbO₃ as the prototype of R3c-type ferroelectrics and BaTiO₃ as the prototype of perovskite-type ferroelectrics). Under ambient conditions, with a low concentration of oxygen vacancies, both LiNbO_3-δ and BaTiO_3-δ become metallic; the polar distortions are reduced in both compounds but do not disappear. However, under pressures, the polar distortions in BaTiO_3-δ are completely suppressed and the system remains metallic with itinerant electron uniformly distributed on Ti atoms. By contrast, the polar distortions in LiNbO_3-δ increase under pressures and above a critical pressure, the system turns into an insulating state. The increased polar displacements in LiNbO_3-δ reduce the band width even though the overall volume decreases under pressures, which localizes the itinerant electrons onto a defect state and eventually leads to a metal-insulator transition.