Influence of the stereochemically active bismuth lone pair structure on ferroelectricity and photocalytic activity of Aurivillius phase Bi2WO6

Abstract

Ab initio electronic structure calculations are reported for the simplest (layered) Aurivillius compound, Bi${}_{2}$${\mathit{WO}}_{6}$, elucidating the role of the asymmetric Bi $6s$ lone-pair structure on the stability of the ferroelectric phase and the band structure. In contrast with the conventional view, where the nature of the para- to ferroelectric transition is explained by the formation of a short Bi-O bond to the perovskite layer, we show that the phase transition is accompanied by cooperative distortions of the asymmetric oxygen arrangements around the bismuth with the formation of particular short Bi-O bonds within the Bi${}_{2}$${O}_{2}$ fluorite sheets. These changes are intimately connected to electronic properties, since while the high-temperature paraelectric phase possesses an indirect band gap, an unusual direct band gap is found at the zone center of the Brillouin zone for ferroelectric Bi${}_{2}$${\mathit{WO}}_{6}$.