Oxygen stabilization in YBa₂Cu₃O_6+δ:Ta₂O₅sintered in air

Abstract

YBa 2 Cu 3 O 6+δ pellets doped with different amounts of Ta 2 O 5 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 Ta 2 O 5 content, the onset temperature differs marginally. The best results are obtained for a Ta 2 O 5 concentration of 1 wt% for which these parameters are 2.5 and 92 K, respectively. On the other hand, air grown single crystals of YBa 2 Cu 3 O 6+δ doped with Ta 2 O 5 need a post growth 48 h anneal in air at 500 • 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 BaTa 2 O 6 and YBa 2 TaO 6 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 (Ta 5+ ). This interpretation is supported by oxygen titration analysis and thermogravimetry measurements on these samples.