  The atomic configuration of phases and their interfaces is fundamental to
materials design and engineering. Here, we unveil a novel crystalline
interface, whose formation is driven by energetic influences that compete in
determining the orientation of an orthorhombic perovskite film. Observed within
LaVO$_3$ grown on $(101)_{\text{orth}}$ DyScO$_3$, the film resolves this
competition by switching its orientation at an atomically-flat plane within its
volume, not at the film-substrate interface. Remarkably, at either side of this
switching plane, characteristic orthorhombic distortions tend to zero to couple
mismatched atomic structures. The resulting boundary is highly energetic; by
using second-principles modeling to map the phase space of film growth, we show
how its formation requires structural relaxation of an entire film grown beyond
a significant critical thickness. This structural-energetic phenomenon opens
avenues towards engineering new functional interfaces, offering the opportunity
of contacting distinct phases of one compound that would never otherwise
coexist.
