Domains and ferroelectric switching pathways inCa3Ti2O7from first principles

Ferroelectrics that allow a coupling between the polarization and another order parameter are of great interest because they could make the electric field control of nonpolar order parameters possible. In recent years, ``hybrid improper''' ferroelectrics - materials where the polarization couples to two different structural distortions - have emerged as a possible way to realize this goal. Theoretical predictions of hybrid improper ferroelectricity in layered perovskite materials were followed by its first experimental realization in Ca${}_{3}$Ti${}_{2}$O${}_{7}$ in 2015. However, the precise pathway by which the polarization reverses in this material during ferroelectric switching remains an open question. The authors address this question and lay the groundwork for understanding the unexpectedly complex domain structure of Ca${}_{3}$Ti${}_{2}$O${}_{7}$, consisting of a network of multiple types of domain walls and topological defects.