Domains and ferroelectric switching pathways in Ca3Ti2 O7 from first principles

Hybrid improper ferroelectricity, where an electrical polarization can be induced via a trilinear coupling to two nonpolar structural distortions of different symmetries, recently was demonstrated experimentally in the n=2 Ruddlesden-Popper compound Ca3Ti2O7. In this paper we use group theoretic methods and first-principles calculations to identify possible ferroelectric switching pathways in Ca3Ti2O7. We identify low-energy paths that reverse the polarization direction by switching via an orthorhombic twin domain or via an antipolar structure. We also introduce a chemically intuitive set of local order parameters to give insight into how these paths are relevant to ferroelectric switching nucleated at domain walls. Our findings suggest that switching may proceed via more than one mechanism in this material.