scale and tailored to have desired properties. Modern synthesis techniques, for instance, can exploit this deeper understanding of structure -property relationships and fabricate epitaxial superlattices composed of alternating layers, each with their own functional property and the complete stack operating as a highly complex and self-contained device. [ 4,5 ] The ideal perovskite structure has cubic symmetry characterized by BO 6 octahedra with equal B-O bond angles/distances in all directions. However, differences in the ionic radii of the A and B atoms lead to tilts and rotations of the octahedra and lowering of structural symmetry. Glazer [ 6 ] devised a notation to describe these tilts along the three principle crystallographic directions and tabulated the families of half-order peaks associated with various lowered structural symmetries. In epitaxial fi lms, the lattice mismatch between the fi lm and the substrate results in differing in-plane vs. out-of-plane bond angles/distances; and/or to a change in the pattern of BO 6 tilts/rotations. These structural distortions occur commonly in perovskites and while several theoretical studies have highlighted their impact on the functional properties, [7][8][9][10]