Highly Tunable Ferroelectricity in Hybrid Improper Ferroelectric Sr₃Sn₂O₇

The successful theoretical prediction and experimental demonstration of hybrid improper ferroelectricity (HIF) provides a new pathway to couple octahedral rotations, ferroelectricity, and magnetism in complex materials. To enable technological applications, an HIF with a small coercive field is desirable. Sr₃Sn₂O₇ single crystals are successfully grown, and it is discovered that they exhibit the smallest electric coercive field at room temperature among all known HIFs. Furthermore, it is demonstated that a small external stress can repeatedly erase and re-generate ferroelastic domains. In addition, using in-plane piezo-response force microscopy, abundant charged and neutral domain walls are charaterized. The observed small electrical and mechanical coercive field values are in accordance with the results of the first-principles calculations on Sr₃Sn₂O₇, which show low energy barriers for both 90° and 180° polarization switching compared to those in other experimentally demonstrated HIFs. The findings represent an advancement towards the possible technological implemetation of functional HIFs.