Precursor Evolution and Nucleation Mechanism of YBa2Cu3Ox Films by TFA Metal−Organic Decomposition

We describe the conversion of yttrium, barium, and copper trifluoracetate-derived solid precursors to epitaxial YBa 2 Cu 3 O x superconducting ceramics on (001)-oriented LaAlO 3 substrates. Transmission electron microscopy, electron energy loss spectroscopy, energy-dispersive X-ray analysis, and X-ray diffraction are used to characterize the reaction path and nucleation mechanism yielding high critical current YBa 2 Cu 3 O 7 . Our results show that the pyrolysis of the trifluoracetate solutions yields a nanostructured, partially amorphous Ba 1- x Y x F 2+ x matrix having a Ba/Y ratio close to 2, with homogeneously dispersed CuO nanoparticles. Upon heating, the chemical trajectory of the fluoride matrix and the overall microstructural evolution of the ceramic precursor prior to YBa 2 Cu 3 O 7 nucleation is driven by the decomposition and oxidation of this solid solution. The Y solid solubility decreases with temperature yielding Y 2 O 3 which reacts with the CuO particles forming Y 2 Cu 2 O 5 at about 700 °C. In addition, electron energy loss spectroscopy reveals a high oxygen concentration and almost no Y in the matrix quenched from 795 °C, at a stage where the YBa 2 Cu 3 O x phase still forms disconnected 50−100 nm thick islands spaced by 1−2 μm. The observed evolution from Ba 1- x Y x F 2+ x to a barium oxyfluoride mostly occurs prior to the heteroepitaxial nucleation of YBa 2 Cu 3 O x at about 700 °C. Hence, a microstructural scenario is defined which favors competitive nucleation growth between heteroepitaxial YBa 2 Cu 3 O x and bulk Y 2 Cu 2 O 5 . X-ray diffraction pole-figure analysis reveals that the oxyfluoride phase is heavily textured, exhibiting two epitaxial relationships with the (001)-LaAlO 3 substrate: (001)OF//(001)LaAlO 3, [110]OF//[100]LaAlO 3 and (111)OF//(001)LaAlO 3, [110]OF//[100]LaAlO 3 (OF stands for oxyfluoride). High-resolution observations of the growth front support that (111)-oriented oxyfluoride regions provide low-barrier nucleation sites for c -axis-oriented YBa 2 Cu 3 O x on the buried (001)-LaAlO 3 substrate. However, owing to its high mismatch, this orientation only represents roughly 15% percent of the total OF volume. Considering that the nucleation of YBa 2 Cu 3 O x is confined to those regions, this would lead to an anomalously large internuclei spacing, as is indeed observed, favoring the formation of large YBa 2 Cu 3 O x grains and films with a low mosaic spread.