Engineering Symmetry Breaking Interfaces by Nanoscale Structural-Energetics in Orthorhombic Perovskite Thin Films

The atomic configuration of phases and their interfaces is fundamental to materials design and engineering. Here, we unveil a transition metal oxide interface, whose formation is driven by energetic influences - epitaxial tensile strain versus oxygen octahedra connectivity - that compete in determining the orientation of an orthorhombic perovskite film. We study this phenomenon in a system of LaVO$_3$ grown on (101) DyScO$_3$, using atomic-resolution scanning transmission electron microscopy to measure intrinsic markers of orthorhombic symmetry. We identify that the film resolves this energetic conflict by switching its orientation by 90 degrees at an atomically-flat plane within its volume, not at the film/substrate interface. At either side of this "switching plane", characteristic orthorhombic distortions tend to zero to couple mismatched oxygen octahedra rotations. The resulting boundary is highly energetic, which makes it a priori unlikely; by using second-principles atomistic modeling, we show how its formation requires structural relaxation of an entire film grown beyond a critical thickness measuring tens of unit cells. The switching plane breaks the inversion symmetry of the Pnma orthorhombic structure, and sharply joins two regions, a thin intermediate layer and the film bulk, that are held under different mechanical strain states. By therefore contacting two distinct phases of one compound that would never otherwise coexist, this alternative type of interface opens new avenues for nanoscale engineering of functional systems, such as a chemically-uniform but magnetically inhomogeneous heterostructure.