Precipitates in YBa2Cu3O7−δ thin films annealed at low oxygen partial pressure

We have studied the precipitates in YBa2Cu3O7−δ(YBCO) thin films grown by the BaF2 process in pO2=4 Torr and 700 °C. While stoichiometric films result in BaCuO2 surface precipitates, we have found Y2Cu2O5 precipitates embedded in the matrix of the same film. Off stoichiometric films with Ba/Y<1.5 have a precipitate-free surface but with higher abundance of Y2Cu2O5 in the film matrix. The estimated densities of the two precipitates favor a stoichiometric YBCO film matrix. This behavior is not explainable in terms of phase equilibria and is attributed to kinetic effects. The electrical properties of the films degrade as the Ba/Y ratio deviates from 2.00.