Epitaxial strain effects on layered polar oxides from first-principles

Epitaxial strain is a powerful tool to generate ferroelectric phases in thin films owing to polarization-strain coupling. The coupling of the oxygen rotations to strain can also be exploited to realize oxygen rotation-sensitive properties such as metal-insulator transitions and magnetic reconstructions. Here, we use electronic structure calculations to investigate the effects of biaxial strain on (001) thin films of the hybrid-improper ferroelectric Ca$_{\mathrm{3}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$. Besides the bulk \textit{Cmc}2$_{\mathrm{1}}$ phase, we also find a new phase emerges under both experimentally accessible biaxial compressive and tensile strains. Furthermore, a large change in the dielectric anisotropy of the film is found at the tensile phase boundary, which we propose could be electric field tunable. Our results may offer a route to search for new functionalities in layered-perovskite oxides.