Secondary phases in (001)RBa2Cu3O7-δ epitaxial thin films

The composition and orientation of secondary phases in epitaxial (001)RBa2Cu3O7-delta thin films grown by MOCVD were investigated by HREM, XRD, SEM, and other techniques. The observed secondary phases often did not correspond to those expected from equilibrium phase diagrams of bulk R-Ba-Cu-O systems. No evidence was found for the presence of R2BaCuO5 or BaCuO2, which are the equilibrium phases with bulk RBa2Cu3O7-delta. In contrast, inclusions of well-oriented R2CuO4 (R = Gd), R2O3 (R = Lu, Ho, Y), R2Cu2O5 (R = Lu), Ba2CuO3, and BaCu3O4 were observed, depending on the films' stoichiometry. It is concluded that the formation of coherent or semicoherent interfaces between embedded inclusions and the surrounding matrix is critical for a stabilization of otherwise nonequilibrium oxide phases in epitaxial films. The crystal structure of unstable-in-bulk cuprate BaCu3O4 was determined using electron nanodiffraction of the films' cross sections.