•Continuously tunable epitaxial strain is achieved beyond substrate limitations•Continuous O-R-T phase transition in BiFeO3 has been obtained on a single substrate•An R/T morphotropic phase boundary is designed on SrTiO3 and NdScO3 substrates 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. The limitation of commercial 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 epitaxial strain in oxide heterostructures beyond substrate limitations by inserting an interface layer through tailoring of its gradual strain relaxation. Taking BiFeO3 as a model system, we demonstrate the introduction of an ultrathin interface layer that allows the creation of desired strain states that can induce phase transition and stabilize a super-tetragonal phase as well as morphotropic phase boundaries, overcoming substrate limitations. Continuously tunable strain from tension to compression can be generated by precisely adjusting the interface layer thickness, enabling the achievement of continuous orthorhombic–rhombohedral-like–tetragonal-like phase transition. This proposed route could be extended to other oxide heterostructures, providing a platform for creating exotic phases and emergent phenomena. The limitation of commercial 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 epitaxial strain in oxide heterostructures beyond substrate limitations by inserting an interface layer through tailoring of its gradual strain relaxation. Taking BiFeO3 as a model system, we demonstrate the introduction of an ultrathin interface layer that allows the creation of desired strain states that can induce phase transition and stabilize a super-tetragonal phase as well as morphotropic phase boundaries, overcoming substrate limitations. Continuously tunable strain from tension to compression can be generated by precisely adjusting the interface layer thickness, enabling the achievement of continuous orthorhombic–rhombohedral-like–tetragonal-like phase transition. This proposed route could be extended to other oxide heterostructures, providing a platform for creating exotic phases and emergent phenomena. 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Here we report a route for imposing continuously tunable, large biaxial strain in oxide heterostructures beyond substrate limitations via the introduction of an interface layer. BiFeO3 (BFO) is a room-temperature multiferroic material that possesses coupled ferroelectric and antiferromagnetic orders.47Wang J. Neaton J.B. Zheng H. Nagarajan V. Ogale S.B. Liu B. Viehland D. Vaithyanathan V. Schlom D.G. Waghmare U.V. et al.Epitaxial BiFeO3 multiferroic thin film heterostructures.Science. 2003; 299: 1719-1722Crossref PubMed Scopus (5216) Google Scholar, 48Catalan G. Scott J.F. Physics and applications of bismuth ferrite.Adv. Mater. 2009; 21: 2463-2485Crossref Scopus (3287) Google Scholar, 49Spaldin N.A. Ramesh R. Advances in magnetoelectric multiferroics.Nat. Mater. 2019; 18: 203-212Crossref PubMed Scopus (620) Google Scholar The parent ground state of this material is a rhombohedral structure (with lattice parameter ∼3.96 Å).47Wang J. Neaton J.B. Zheng H. Nagarajan V. Ogale S.B. Liu B. 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Nanotechnol. 2015; 10: 972-979Crossref PubMed Scopus (111) Google Scholar However, this strain-induced R-T phase transition and the stabilization of the metastable T-phase BFO (T-BFO, with in-plane lattice parameter a ∼3.66 Å, out-of-plane lattice parameter c ∼4.65 Å, and large c/a ratio ∼1.27), can be achieved only on substrates that provide large compressive strains exceeding −4.3% at room temperature,9Zeches R.J. Rossell M.D. Zhang J.X. Hatt A.J. He Q. Yang C.-H. Kumar A. Wang C.H. Melville A. Adamo C. et al.A strain-driven morphotropic phase boundary in BiFeO3.Science. 2009; 326: 977-980Crossref PubMed Scopus (940) Google Scholar,57Hatt A.J. Spaldin N.A. Ederer C. Strain-induced isosymmetric phase transition in BiFeO3.Phys. Rev. B. 2010; 81: 054109Crossref Scopus (230) Google Scholar whereas films on substrates with epitaxial strain of −4.3% to +1% comprise R-BFO,9Zeches R.J. Rossell M.D. Zhang J.X. Hatt A.J. He Q. Yang C.-H. Kumar A. Wang C.H. Melville A. 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In this study, taking BFO as a model system, we demonstrate that the insertion of an ultrathin interface layer (as thin as 7 unit-cells [u.c.]) allows the creation of a desired strain overcoming substrate limitations. Using a combination of X-ray diffraction (XRD) with reciprocal space maps (RSMs), scanning probe microscopy, and transmission electron microscopy (TEM), we find that the use of this approach not only enables the growth of high-quality BFO epitaxial thin films, but also provides the ability to generate a desired strain that can induce phase transition and stabilize the T phase as well as the morphotropic phase boundary, whether on compressive or tensile strain substrates. Moreover, continuously tunable strain cutting across tension to compression can be imparted by accurately adjusting the thickness of an interface layer, enabling the realization of continuous O-R-T phase transition in BFO on a single substrate. Based on previous first-principle calculations and phase field simulations,9Zeches R.J. Rossell M.D. Zhang J.X. Hatt A.J. He Q. Yang C.-H. Kumar A. Wang C.H. Melville A. Adamo C. et al.A strain-driven morphotropic phase boundary in BiFeO3.Science. 2009; 326: 977-980Crossref PubMed Scopus (940) Google Scholar,56Chu K. Jang B.-K. Sung J.H. Shin Y.A. Lee E.-S. Song K. Lee J.H. Woo C.-S. Kim S.J. Choi S.-Y. et al.Enhancement of the anisotropic photocurrent in ferroelectric oxides by strain gradients.Nat. Nanotechnol. 2015; 10: 972-979Crossref PubMed Scopus (111) Google Scholar,57Hatt A.J. Spaldin N.A. Ederer C. Strain-induced isosymmetric phase transition in BiFeO3.Phys. Rev. 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Mater. 2005; 17: 2069Crossref Scopus (67) Google Scholar we expect to overcome substrate limitations to impart epitaxial strain for the formation of the metastable T-BFO, not only on LAO substrate, but also on LSAT and STO substrates and even on NSO (Figure 1B). Using pulsed laser deposition (PLD), we grew a series of epitaxial BFO thin films and BFO/CCMO heterostructures on (001) LAO, (001) LSAT, (001) STO, and (110)O NSO substrates (please note that all substrates used in this study are pseudocubic (001)-oriented single crystals). XRD 2θ–ω scans of 14-nm-thick BFO films grown on these bare substrates illustrate the epitaxial growth of BFO and the creation of the T phase on LAO, R phase on LSAT and STO, and O phase on NSO (Figure 1C). These results are consistent with previous theoretical and experimental studies.9Zeches R.J. Rossell M.D. Zhang J.X. Hatt A.J. He Q. Yang C.-H. Kumar A. Wang C.H. Melville A. 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Yu P. Yang S.Y. Wang C.H. Chu Y.H. Martin L.W. et al.Large field-induced strains in a lead-free piezoelectric material.Nat. Nanotechnol. 2011; 6: 98-102Crossref PubMed Scopus (274) Google Scholar we asked whether it could also evolve to an R/T mixed phase and design a morphotropic phase boundary by increasing the film thickness. Atomic force microscopy (AFM) measurement was performed on a series of BFO/CCMO heterostructures with 7-, 25-, and 50-nm-thick BFO and 27-nm-thick CCMO on LSAT, STO, and NSO substrates. The thickness-dependent evolution of surface morphologies (Figure 2) revealed high-quality epitaxial growth of BFO films, showing atomically flat morphologies (root-mean-square roughness <0.3 nm). Furthermore, the 1 u.c. in height terraces (Figure 2A) indicate the formation of pure T phase in 7-nm-thick BFO on STO, whereas the bright-contrast matrix and dark-contrast stripes in 25-nm-thick film (Figure 2B) suggest the emergence of the R phase that coexists with the T phase due to relaxation of the epitaxial strain with increased thickness. Further strain relaxation in thicker films (50 nm) results in further increase of the R-phase fraction (Figure 2C). The distinct thickness-dependent phase evolution is observed on NSO (Figures 2D–2F) and LSAT (Figure S2) substrates as well. It is worth mentioning that we report here for the f