Review of experimental progress of hybrid improper ferroelectricity in layered perovskite oxides

The primary order parameter of hybrid improper ferroelectricity (HIF) is not spontaneous polarisation but nonpolar modes such as oxygen octahedral rotations (OORs), antipolar, or even Jahn–Teller distortions; therefore, the HIF mechanism may be applied as an effective pathway to tune electronic bandgaps, control orbitals, and create multiferroicity. Most of the current experimental research on HIF is focused on layered perovskite oxides;...