TL-O CHARGE RESERVOIR CHANGES ON ANNEALING TL-1223 CRYSTALS

Two TlCa2Ba2Cu3O9-delta (Tl-1223) single crystals have been sequentially annealed in flowing oxygen or nitrogen and the complete crystal structure determined by least-squares refinement of Mo K-alpha-X-ray diffraction data. Small atomic displacements are consistent with a model in which small amounts of Tl1+ present in the Tl-O layer are reversibly oxidized to Tl3+ during mild oxygen anneals. Further, the disordered location of the Tl position off the four-fold, as determined by the usual least-squares refinement of X-ray data, appears to be related to the amount of oxygen present in the Tl-O layer in the particular crystal. Despite substantial changes in oxygen content in the Tl-O layers with annealing, Meissner data show very modest shifts in the superconducting transition temperature. T(c)'s are 117 K after extended oxygen anneals and 121 K for nitrogen anneals, suggesting little change in the carrier concentration in the Cu-O2 planes.