ON TL-1212 SUPERCONDUCTOR SINGLE-CRYSTALS - STRUCTURE AND MAGNETISM CHANGES ON ANNEALING

The complete crystal structure of several single crystals with nominal stoichiometry TlBa2CaCu2O7 (Tl-1212) has been determined by least-squ ares refinement of MoK alpha X-ray diffraction data before and after s equential anneals in flowing and/or oxygen. Prior work, primarily on T lBa2Ca2Cu3Ox, suggested thar both Tl1+ and Tl3+ are present in the cha rge reservoir Tl-O layer(s). The relative amounts could be reversibly altered by mild oxidation or reduction anneals without significant cha nges in the superconducting onset temperature, T-c, consistent with ma intaining a hole concentration near the optimum doping. These anneals produced small reversible atomic displacements within the unit cell. I n contrast, the atomic positional changes in the Tl-1212 crystals upon such annealing are just at the detectability level, and although all the crystals studied belong to this structure type, the mean values in some of these parameters grouped by starting growth compositions resu lt in differences at the 95% confidence level. Meissner data show shar pening of the transition of the as-grown crystals with annealing, but T-c changes are much smaller than observed in Tl-1212 powders. (C) 199 7 Elsevier Science B.V.