Microscopic Origin of the Giant Ferroelectric Polarization in Tetragonal-likeBiFeO3

We report direct experimental evidence for a room-temperature, $\ensuremath{\sim}130\text{ }\text{ }\ensuremath{\mu}\mathrm{C}/{\mathrm{cm}}^{2}$ ferroelectric polarization from the tetragonal-like ${\mathrm{BiFeO}}_{3}$ phase. The physical origin of this remarkable enhancement of ferroelectric polarization has been investigated by a combination of x-ray absorption spectroscopy, scanning transmission electron microscopy, and first principles calculations. A large strain-induced Fe-ion displacement relative to the oxygen octahedra, combined with the contribution of Bi $6s$ lone pair electrons, is the mechanism driving the large ferroelectric polarization in this tetragonal-like phase.