Oxygen stabilization in YBa2Cu3O6+delta : Ta2O5 sintered in air

YBa2Cu3O6+delta 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 ha ve been observed to exhibit superconductivity onset at 91 K, even without p ost sintering oxygen annealing. While the width of the transition varies wi th the Ta2O5 content, the onset temperature differs marginally. The best re sults are obtained for a Ta2O5 concentration of 1 wt% for which these param eters are 2.5 and 92 K, respectively. On the other hand, air grown single c rystals of YBa2Cu3O6+delta doped with Ta2O5 need a post growth 48 h anneal in air at 500 degrees C to observe a broad superconducting transition at 80 K (onset temperature). Powder x-ray diffraction analysis shows the stabili zation of the orthorhombic 123 phase and the presence of the BaTa2O6 and YB a2TaO6 phases in small amounts. These results are interpreted in terms of t he incorporation and stabilization of the stoichiometric excess of oxygen i n the presence of pentavalent Ta (Ta5+). This interpretation is supported b y oxygen titration analysis and thermogravimetry measurements on these samp les.