Several families of layered perovskite oxide ferroelectrics exhibit a
coupling between polarization and structural order parameters, such as
octahedral rotation distortions. This coupling provides opportunities for novel
electric field-based manipulation of material properties, and also stabilizes
complex domain patterns and domain wall vortices. Amongst layered perovskites
with such coupled orders, the Aurivillius-phase oxides SrBi2βB2βO9β
(B=Ta, Nb) are well-known for their excellent room temperature ferroelectric
performance. This work combines group theoretic analysis with density
functional theory calculations to examine the ferroelectric switching processes
of SrBi2βB2βO9β. Low-energy two-step ferroelectric switching paths are
identified, with polarization reversal facilitated by structural order
parameter rotations. Analysis of the domain structure reveals how the relative
energetics of the coupled order parameters translates into a network of several
distinct domain wall types linked by domain wall vortex structures. Comparisons
are made between the ferroelectric switching and domain structure of
SrBi2βB2βO9β and those of the layered n=2 Ruddlesden-Popper hybrid
improper ferroelectrics. The results provide new insight into how ferroelectric
properties may be optimized by engineering the complex crystal structures of
Aurivllius-phase oxides