Catalytically active (001) ceria substrates or buffers are used to modify the structure of the epitaxial high temperature superconductor YBa.sub.2Cu.sub.3O.sub.7. The catalytically active substrate has a small lateral grain size, typically less than 50 nm, to provide a high density of nucleation sites, at some of which nucleate a previously unknown metastable phase. The modification is achieved by catalytically assisted synthesis of the metastable phase. The new phase, a long-period (3.5-nm) perovskite, intercalates into the YBa.sub.2Cu.sub.3O.sub.7 matrix without negatively affecting the critical temperature of the film. Analysis of electron microscopy and synchrotron X-ray diffraction data allow identification of the phase as a long-period YBa.sub.2Cu.sub.3O.sub.7 derivative formed through short-range cation displacement. The films, from about 100-nm to about 1000-nm thick, exhibit strong enhancement of the critical current density, reaching a maximum of approximately 4.2 MA/cm.sup.2 at 77 K.