Oxygen stabilization in YBa2Cu3O6+δ:Ta2O5 sintered in air

YBa2Cu3O6+δ pellets doped with different amounts of Ta2O5 in the range of 0.1-4 wt% and sintered in air before being cooled to room temperature have been observed to exhibit superconductivity onset at 91 K, even without post sintering oxygen annealing. While the width of the transition varies with the Ta2O5 content, the onset temperature differs marginally. The best results are obtained for a Ta2O5 concentration of 1 wt% for which these parameters are 2.5 and 92 K, respectively. On the other hand, air grown single crystals of YBa2Cu3O6+δ doped with Ta2O5 need a post growth 48 h anneal in air at 500 deg. C to observe a broad superconducting transition at 80 K (onset temperature). Powder x-ray diffraction analysis shows the stabilization of the orthorhombic 123 phase and the presence of the BaTa2O6 and YBa2TaO6 phases in small amounts. These results are interpreted in terms of the incorporation and stabilization of the stoichiometric excess of oxygen in the presence of pentavalent Ta (Ta5+). This interpretation is supported by oxygen titration analysis and thermogravimetry measurements on these samples. (author)