Ferroelectricity in Low-Permittivity SrZrO₃ Epitaxial Films

Bulk SrZrO3 has an orthorhombic perovskite structure (Pbnm) with a central symmetry but exhibits a low dielectric constant. In this study, we reported a room-temperature ferroelectric SrZrO3 thin film with a low dielectric constant, induced by compressive strain from the SrTiO3 substrate. The presence of an out-of-phase boundary structure allows SrZrO3 with a large lattice mismatch to grow epitaxially on SrTiO3 substrates. The apparent atomic displacements in the lattice are revealed by scanning transmission electron microscopy imaging. Compressive strain induces an orthorhombic-to-monoclinic phase transition, evidenced by both the atomic structure obtained from the annular bright-field image and the structure optimized with first-principles calculation. Further first-principles calculations demonstrated that compressive strain causes a polar displacement of the Zr atom inside the ZrO6 octahedron, which is the origin of the ferroelectricity. Our work shows a way to design novel ferroelectrics from conventional non-ferroelectric materials, which is of significant importance to robust, stable, and lead-free ferroelectrics applications.