Epitaxial strain engineering is a powerful approach to generate and tailor collective phenomena and new functionalities in thin film oxides with layered habits, e.g. those of the homologous series An+1BnO3n+1 (n=1-∞). Although large epitaxial strains are believed to always induce ferroelectricity (FE), here we demonstrate that biaxial strain induces an unanticipated polar-to-nonpolar (P-NP) structural transition in (001) thin films of layered hybrid-improper ferroelectrics with the n=2 Ruddlesden-Popper structure at experimentally accessible biaxial compressive and tensile strains [1]. We show in detail for Ca3Ti2O7 that the origin of the P-NP transition originates from the interplay of trilinear-related lattice mode interactions active in the layered oxides, and those interactions are directly strain tunable. We use this understanding to show that the P-NP transition also occurs in (001) Ca3Mn2O7 and Sr3Zr2O7 thin films. Last, we propose a design principle for selecting the required A and B cation chemistries that ensure strained A3B2O7 (001) oriented films exhibit P-NP transitions [2], which we substantiate with density functional calculations. Our results call for a careful re-examination of the role of strain-polarization coupling in FE films with nontrivial anharmonicities and offer a route to search for new functionalities in oxides through multimode coupling.
References
[1] X.-Z. Lu and J.M. Rondinelli, Nature Materials 15, 951-955 (2016).
[2] X.-Z. Lu and J.M. Rondinelli, Advanced Functional Materials (2017). DOI: 10.1002/adfm.201604312