Strain Programming of Oxygen Octahedral Symmetry in Perovskite Oxide Thin Films

Abstract

The collective rotations of oxygen octahedra play an important role in determining the physical properties of functional perovskite oxides. The epitaxial strain can serve as an effective means to modify the oxygen octahedral symmetry (OOS), i.e., oxygen octahedral rotation or tilt (OOR/OOT). However, the strain-OOS coupling that may alter the details of the OOS, thereby the physical properties, has not been fully understood. In this work, it is demonstrated that epitaxial strain can not only induce a structural phase transition but also precisely tune the degree of OOR. The correlated metal CaNbO₃, which is orthorhombic, is studied by growing as epitaxial thin films. By imposing epitaxial strain, it is found that the film undergoes a structural phase transition from orthorhombic to tetragonal upon fully straining (i.e., from a⁺b⁻b⁻ to a⁰a⁰c⁻). In unstrained films, the octahedral rotation along the c-axis is as large as 15.7° that can be tuned to 6.6° by strain. This finding offers a general approach to manipulating OOR/OOT via strain engineering in complex oxide heterostructures.