  The limitation of commercially available single-crystal substrates and the
lack of continuous strain tunability preclude the ability to take full
advantage of strain engineering for further exploring novel properties and
exhaustively studying fundamental physics in complex oxides. Here we report an
approach for imposing continuously tunable, large epitaxial strain in oxide
heterostructures beyond substrate limitations by inserting an interface layer
through tailoring its gradual strain relaxation. Taking BiFeO3 as a model
system, we demonstrate that the introduction of an ultrathin interface layer
allows the creation of a desired strain that can induce phase transition and
stabilize a new metastable super-tetragonal phase as well as morphotropic phase
boundaries overcoming substrate limitations. Furthermore, continuously tunable
strain from tension to compression can be generated by precisely adjusting the
thickness of the interface layer, leading to the first achievement of
continuous O-R-T phase transition in BiFeO3 on a single substrate. This
proposed route could be extended to other oxide heterostructures, providing a
platform for creating exotic phases and emergent phenomena.
