Pressure-induced metal-insulator transition in oxygen-deficient ferroelectrics

Pressure and oxygen vacancies are usually considered as deleterious for ferroelectric materials because both tend to reduce their polarization. Extending our previous work (Phys. Rev. Materials 3, 054405 (2019)), we perform first-principles calculations to show that in oxygen-deficient LiNbO_3−δ, pressure induces a novel metal-insulator transition around 8-9 GPa. Strong polar displacements are robust in both metallic and insulating LiNbO_3−δ. The transition arises from the change of an oxygen vacancy defect state. Pressure increases the polar displacements of LiNbO_3−δ, which reduces the band width of the defect state and eventually turns it into an in-gap state. In the insulating phase, the in-gap state is further pushed away from the conduction band edge under pressure, which increases the fundamental gap. Our work shows that for LiNbO₃-type strong ferroelectrics, combining oxygen vacancies and pressure can lead to new phenomena and potential functions, in contrast to the harmful effects occurring to perovskite ferroelectric oxides such as BaTiO₃.