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

Sillén-Aurivillius layered perovskite oxyhalides BiO ( = Nb, Ta; = 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 BiNbOBr (space group: 2), revealing that the intralayer polarization is not based on the reported NbO octahedral tilting but is derived from the stereochemically active Bi lone pair electrons (LPEs) and the octahedral off-centering of Nb cations. The revised structure (space group: ) has additional interlayer polarizations (calculated: 0.6 μC/cm), in agreement with recent experiments on BiNbOBr. The occurrence of polarization due to stereochemically active LPEs and Nb-site off-centering is similar to Aurivillius-type ferroelectrics (e.g., BiWO), with comparable spontaneous polarizations in the in-plane direction (calculated: 43.5 μC/cm). This, together with the out-of-plane polarization, indicates that Sillén-Aurivillius compounds have great potential as ferroelectric materials.