Domain boundaries in hybrid improper ferroelectric layered perovskites

Coupling or competition between spin, charge, orbital, and structural order parameters can lead to novel properties and functionality in materials. In the hybrid improper ferroelectric n=2 Ruddlesden-Popper oxides, the polarization is coupled to two octahedral rotation distortions of different symmetries. This coupling not only enables new functionality, but also stabilizes a fascinating domain topology of different types of domain boundaries coupled together at domain wall vortices and antivortices. However, the precise way in which the multiple order parameters in hybrid improper ferroelectrics evolve across the domain boundaries, and their atomic-scale structure, is still an open question. In this talk, I will utilize a combination of group theoretic methods, Landau theory, and density functional theory calculations to present scenarios for the local structure at domain boundaries in the hybrid improper ferroelectric Ca₃Ti₂O₇ and related A₃B₂O₇ materials.