Improper geometric ferroelectricity at the monolayer limit

Improper ferroelectrics, in which electric polarization emerges as a secondary effect of primary order parameters such as nonpolar structural distortions, are theoretically predicted to exhibit no intrinsic thickness limit. Yet, this prediction is inconsistent with experiment where ferroelectricity is observed to disappear in improper ferroelectric films under six formula units thick. We transcend this limitation by moving beyond the conventional epitaxial design paradigm that focuses largely on in-plane lattice mismatch and chemical bonding. By considering structural compatibility in the out-of-plane direction, we realize undiminished improper ferroelectricity at the monolayer limit. Our findings confirm the theoretically predicted absence of a critical thickness for improper ferroelectrics and establish a broadly applicable strategy for designing additional ultrathin ferroelectric materials.