Ferroelectric switching pathways, domain structure, and domain wall vortex in SrBi₂(Ta,Nb)₂O₉

Aurivillius-phase oxides SrBi₂B₂O₉ (B=Ta, Nb) are room temperature ferroelectrics which are well known for their fatigue resistance, low coercive fields, and potential application in ferroelectric random-access memory. Previous work has shown that the trilinear coupling between the electrical polarization and two octahedral rotation distortions of different symmetries plays a key role in the structural phase transition sequence and stabilization of the ferroelectric phase in SrBi₂Ta₂O₉. However, the implications of the trilinear coupling on the ferroelectric switching mechanism in these materials remain to be understood. In this work, we use the group theoretic analysis and density functional theory calculations to enumerate and explore the energetics of ferroelectric switching paths in SrBi₂B₂O₉ and identify several low-energy one- and two-step ferroelectric switching pathways. Moreover, we show how the relative energy barriers of these switching paths can provide the insight into the domain structure of these oxides. In particular, our findings indicate that three-fold domain wall vortices are energetically favorable in SrBi₂B₂O₉ which corroborates experimental reports of the domain structure in SrBi₂Ta₂O₉. This work advances our fundamental understanding of the technologically important family Aurivillius oxides.