CRYSTAL-STRUCTURE AND CATION STOICHIOMETRY OF SUPERCONDUCTING TL2BA2CUO6+DELTA SINGLE-CRYSTALS

High quality single crystals of Tl2Ba2CuO6+delta have been obtained by a CuO self-flux method. The synthesized crystals have an average chemical composition of Tl1.85(4)Ba1.94(4)Cu1.15(2)O6+delta as determined by electron probe microanalysis (EPMA), suggesting a possible substitution of Cu for Tl. Single-crystal X-ray diffraction experiments and structure refinement show that the crystal exhibits tetragonal symmetry (space group I4/mmm), with lattice parameters a = 3.8608 (4) angstrom and c = 23.1332 (9) angstrom. Site occupancy refinements indicated completely occupied Ba and Cu sites within the structure, and a deficiency in the scattering power of the Tl site. The final structure refinement corresponds to a composition Tl1.87(2)Ba2Cu1.13(2)O6+delta, which is consistent with the EPMA results. The quality of the diffraction data allow Tl and O (which comprise the Tl2O2 layers) to be unambiguously assigned to split sites of the type xOz, as opposed to xxz sites. There is no evidence for long-range ordering of the displacements of the atoms in the T1202 layers of this crystal. The as-grown single crystals have a superconducting transition temperature, T(c), of 12.4 K. The T(c) of the as-grown crystals could be enhanced via low temperature annealing in a reducing atmosphere.