Secondary Phases in (001)RBa2Cu3O7-δ Epitaxial Thin Films

The composition and orientation of secondary phases in epitaxial (001)RBa 2 Cu 3 O 7 - δ 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 R 2 BaCuO 5 or BaCuO 2, which are the equilibrium phases with bulk RBa 2 Cu 3 O 7 - δ . In contrast, inclusions of well-oriented R 2 CuO 4 (R = Gd), R 2 O 3 (R = Lu, Ho, Y), R 2 Cu 2 O 5 (R = Lu), Ba 2 CuO 3, and BaCu 3 O 4 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 BaCu 3 O 4 was determined using electron nanodiffraction of the films' cross sections.