Phase evolution of Ba₂YCu₃O6+xfilms during the BaF₂process

High-temperature x-ray diffraction and transmission electron microscopy were used to study the phase evolution and reaction kinetics during the ex situ BaF 2 conversion process to form Ba 2 YCu 3 O 6+ x (Y-213). For this study, precursor films with compositions corresponding to Y-213 were deposited on single crystal SrTiO 3 substrates using an e-beam co-evaporation technique. Two different precursors, (BaF 2 +Y +Cu) and (BaF 2 +Y F 3 +Cu), were used, and the phase evolution in the resulting series of samples was compared. For the (BaF 2 +Y +Cu) films, at intermediate stages of the conversion process, the amorphous precursor partially crystallized into a mixture of a BaF 2 -like oxyfluoride phase ( , where BF denotes BaF 2 ) and CuO. At temperatures close to 700 °C, Y 2 Cu 2 O 5 and a BaF 2 -related superstructure ( , , ) were observed. For this series of films, the ex situ process involved several hydration/oxidation reactions which ultimately transformed the precursor into the Y-213 phase. The overall reaction scheme can be represented as: , where Ba (O x F y ) represents both the BaF 2 -like oxyfluoride and the BaF 2 -related superstructure phases. The growth of the Y-213 phase and the consumption of BaF 2 -like phases were approximately linear in time. The resulting Y-213 films were predominantly c -axis textured, along with smaller volume fractions of a -axis textured and randomly oriented regions. For the (BaF 2 +Y F 3 +Cu) films investigated, we observed a similar evolution of the Ba (O x F y ) phase and the BaF 2 -related superstructure, along with crystallization of CuO; however, Y 2 Cu 2 O 5 (s) was not observed. The Y F 3 component remained amorphous (from x-ray diffraction results) and reacted with Ba (O x F y ), CuO and H 2 O to form Y-213 and Y 2 O 3 . The presence of unreacted Ba (O x F y ), Y 2 O 3 and CuO in the film after prolonged (>2 h) annealing at 735 °C indicates incomplete Y-213 conversion. We conclude that the (BaF 2 +Y F 3 +Cu) precursor was less suitable relative to (BaF 2 +Y +Cu) under the present processing conditions.