Lone-Pair-Induced Intra- and Interlayer Polarizations in Sillén-Aurivillius Layered Perovskite Bi 4 NbO 8 Br.

Sillén-Aurivillius layered perovskite oxyhalides Bi 4 M O 8 X ( 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 Bi 4 NbO 8 Br (space group: P 2 1 cn ), revealing that the intralayer polarization is not based on the reported NbO 6 octahedral tilting but is derived from the stereochemically active Bi 3+ lone pair electrons (LPEs) and the octahedral off-centering of Nb 5+ cations. The revised structure (space group: Ic ) has additional interlayer polarizations (calculated: 0.6 μC/cm 2 ), in agreement with recent experiments on Bi 4 NbO 8 Br. The occurrence of polarization due to stereochemically active LPEs and Nb-site off-centering is similar to Aurivillius-type ferroelectrics (e.g., Bi 2 WO 6 ), with comparable spontaneous polarizations in the in-plane direction (calculated: 43.5 μC/cm 2 ). This, together with the out-of-plane polarization, indicates that Sillén-Aurivillius compounds have great potential as ferroelectric materials.