Multianion induced out-of-plane proper polarization in oxyfluoride Aurivillius Bi2TiO4F2

Abstract

We explore the effect of anion ordering on the ferroelectric properties of the oxyfluoride Aurivillius Bi2TiO4F2 from first-principles calculations. We first identify the phonon instabilities in the high symmetry reference phases and build the energy diagram for the different low symmetry metastable phases coming from these instabilities. We found that the apical ordering of fluorine is more stable than the equatorial one but mixing apical and equatorial ordering together is even more favorable. For each fluorine ordering, the ground state is polar and exhibits a proper in-plane polarization. We also found that the apical ordering can exhibit a proper out-of plane polarization as large as 35 μc/cm2, which is unexpected in layered crystals. This finding opens the possibility to design polarization perpendicular to the stacking layers in ferroelectrics layered perovskites through multianion engineering, a property strongly desired for technological applications like FeRAM devices.