On Tl-1212 superconductor single crystals: structure and magnetism changes on annealing

Abstract

The complete crystal structure of several single crystals with nominal stoichiometry TlBa 2CaCu 2O 7 (Tl-1212) has been determined by least-squares refinement of Mo K α X-ray diffraction data before and after sequential anneals in flowing argon and/or oxygen. Prior work, primarily on TlBa 2Ca 2Cu 3O x , suggested that both Tl 1+ and Tl 3+ are present in the charge reservoir Tl-O layer(s). The relative amounts could be reversibly altered by mild oxidation or reduction anneals without significant changes in the superconducting onset temperature, T c, consistent with maintaining a hole concentration near the optimum doping. These anneals produced small reversible atomic displacements within the unit cell. In 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 result in differences at the 95% confidence level. Meissner data show sharpening of the transition of the as-grown crystals with annealing, but T c changes are much smaller than observed in Tl-1212 powders.