Ferroelectric Switching Paths and Domain Structure of Aurivillius-Phase SrBi₂(Ta,Nb)₂O₉ from First Principles

In the last decade, layered perovskite oxide materials – with crystal structures that interleave perovskite blocks with other structural units – have emerged as a family of materials the combine novel ferroelectric mechanisms with additional functionalities. Amongst layered perovskites, the Aurivillius-phase ferroelectrics have been known for decades for their fatigue resistance and robust ferroelectric properties. More recently, the ferroelectricity in two prominent members of the Aurivillius family, SrBi₂Ta₂O₉ and SrBi₂Nb₂O₉, has been shown to arise from a novel trilinear coupling mechanism between polarization and octahedral rotation distortions. This coupling also has been shown to play a key role in the complex paraelectric-ferroelectric structural phase transition sequences of these materials. However, the implications of the trilinear coupling on the ferroelectric switching mechanism in SrBi₂<i>B</i>₂O₉ (<i>B</i>=Ta, Nb) remains to be understood. In this work, we use group theoretic analysis and density functional theory calculations to enumerate and explore the energetics of ferroelectric switching paths in SrBi₂<i>B</i>₂O₉ and identify low-energy two-step switching pathways facilitated by structural order parameter rotations. Moreover, we show how the relative energy barriers of these switching paths can provide insight into the domain structure of these oxides. In particular, our findings indicate that three-fold domain wall vortices are energetically favorable in SrBi₂<i>B</i>₂O₉. This work provides new insight into the origin of low-energy ferroelectric switching in SrBi₂<i>B</i>₂O₉ and provides strategies to further lower the switching barrier, which is a key parameter for ferroelectric performance.