Precursor evolution and nucleation mechanism of YBa2Cu3Ox films by TFA metal-organic decomposition

J. Gazquez, F. Sandiumenge, M. Coll, A. Pomar, N. Mestres, T. Puig, X. Obradors, Y. Kihn, M. J. Casanove and C. Ballesteros Chem. Mat. 18, 6211-6219 (2006) DOI: 10.1021/cm0617891 We describe the conversion of yttrium, barium, and copper trifluoracetate-derived solid precursors to epitaxial YBa₂Cu₃O_x superconducting ceramics on (001)-oriented LaAlO₃substrates. 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 YBa₂Cu₃O₇. Our results show that the pyrolysis of the trifluoracetate solutions yields a nanostructured, partially amorphous Ba_1-xY_xF_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 YBa₂Cu₃O₇ nucleation is driven by the decomposition and oxidation of this solid solution. The Y solid solubility decreases with temperature yielding Y₂O₃ which reacts with the CuO particles forming Y₂Cu₂O₅ 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 YBa₂Cu₃O_x phase still forms disconnected 50−100 nm thick islands spaced by 1−2 μm. The observed evolution from Ba_1-xY_xF_2+x to a barium oxyfluoride mostly occurs prior to the heteroepitaxial nucleation of YBa₂Cu₃O_x at about 700 °C. Hence, a microstructural scenario is defined which favors competitive nucleation growth between heteroepitaxial YBa₂Cu₃O_x and bulk Y₂Cu₂O₅. X-ray diffraction pole-figure analysis reveals that the oxyfluoride phase is heavily textured, exhibiting two epitaxial relationships with the (001)-LaAlO₃ substrate:  (001)OF//(001)LaAlO₃, [110]OF//[100]LaAlO₃ and (111)OF//(001)LaAlO₃, [110]OF//[100]LaAlO₃ (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 YBa₂Cu₃O_x on the buried (001)-LaAlO₃ substrate. However, owing to its high mismatch, this orientation only represents roughly 15% percent of the total OF volume. Considering that the nucleation of YBa₂Cu₃O_x is confined to those regions, this would lead to an anomalously large internuclei spacing, as is indeed observed, favoring the formation of large YBa₂Cu₃O_x grains and films with a low mosaic spread.