Lone-Pair-Induced Intra- and Interlayer Polarizations in Sillén-Aurivillius Layered Perovskite Bi₄NbO₈Br

Sillén-Aurivillius layered perovskite oxyhalides Bi₄MO₈X (M = Nb, Ta; X = Cl, Br) are of great interest because of their potential as lead-free ferroelectrics in addition to their function as visible-light-responsive photocatalysts. In this work, we revisited the crystal structure of Bi₄NbO₈Br (space group: P2₁cn), revealing that the intralayer polarization is not based on the reported NbO₆ octahedral tilting but is derived from the stereochemically active Bi³⁺ lone pair electrons (LPEs) and the octahedral off-centering of Nb⁵⁺ cations. The revised structure (space group: Ic) has additional interlayer polarizations (calculated: 0.6 μC/cm²), in agreement with recent experiments on Bi₄NbO₈Br. The occurrence of polarization due to stereochemically active LPEs and Nb-site off-centering is similar to Aurivillius-type ferroelectrics (e.g., Bi₂WO₆), with comparable spontaneous polarizations in the in-plane direction (calculated: 43.5 μC/cm²). This, together with the out-of-plane polarization, indicates that Sillén-Aurivillius compounds have great potential as ferroelectric materials.