Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Epitaxial strain, imparted by an underlying substrate, is a powerful pathway to drive phase transitions and alter properties in complex oxides, enabling the creation of new ground states and novel functionalities. To realize these emergent phenomena, the availability of appropriate single-crystal substrates for the growth of high-quality epitaxial oxide films with a desired strain state cannot be overemphasized. However, the limitation of commercial substrates and the lack of continuous strain tunability result in stringent restrictions on the further discovery of novel properties and fundamental physics. Here, we propose a strategy for imposing continuously tunable strain beyond substrate limitations by inserting an interface layer, enabling the achievement of continuous orthorhombic–rhombohedral-like–tetragonal-like phase transition in BiFeO₃ films on a single substrate and the integration of morphotropic phase boundary on different substrates. This work provides a framework for the strain engineering of complex oxides.