Xuezeng Lu1 James Rondinelli1

1, Northwestern University, Evanston, Illinois, United States

Epitaxial strain is a powerful tool to generate ferroelectricity owing to polarization-strain coupling. Local lattice degrees-of-freedom such as rotations of metal-oxygen octahedral also couple to strain, and can be used to tune a material’s oxygen rotational-related properties such as metal-insulator transitions and magnetic reconstruction by strain. Here, we first use electronic structure calculations to investigate the strain effects on (001) thin films of the hybrid-improper ferroelectric (HIF) A3B2O7 compounds. Surprisingly, other than the bulk polar Cmc21 phase, we find a new nonpolar phase becomes the ground state under both experimentally accessible biaxial compressive and tensile strains, which is beyond the people’s believe about the rule of the polarization-strain coupling. Furthermore, the generality of the polar-to-nonpolar (N-NP) transition in HIFs (not only in A3B2O7 compounds) leads us to find a novel route to tune the physical properties that are classified as mechanical, optical and magnetic responses, which we also propose could be electric-field tunable, near the P-NP phase transition boundary. Our results may offer a route to search for new functionalities in hybrid-improper ferroelectrics.