TlO charge reservoir changes on annealing Tl-1223 crystals

Abstract

Two TlCa 2Ba 2Cu 3O 9−δ (Tl-1223) single crystals have been sequentially annealed in flowing oxygen or nitrogen and the complete crystal structure determined by leastsquares refinement of Mo Kα X-ray diffraction data. Small atomic displacements are consistent with a model in which small amounts of Tl 1+ present in the TlO layer are reversibly oxidized to Tl 3+ 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O 2 planes.