SINGLE-CRYSTAL STUDY OF THE BASR-1212 SUPERCONDUCTOR TL1-DELTA(XBASRCA1+XCU2O7)

Single crystals of the 'BaSr' thallium cuprate Tl1+xBaSrCa1-xCu2O7-del ta have been grown for the first time. The structure parameters of thi s tetragonal phase were successively refined using four single-crystal X-ray diffraction data of the same crystal, as-grown [T-c=62K, a=3.82 71(7), c=12.385(3) Angstrom] and after different thermal treatments: t wo annealings under H-2/Ar flow [T-c=90K, a=3.8356(7), c=12.390(3) Ang strom] and one under oxygen pressure P-O2 = 100 bar [T-c=60 K, a=3.823 4 (9), c= 12.384(4) Angstrom]. The corresponding results provided evid ence of a reversible evolution of the cation oxygen distances and of t he thallium splitting depending on the reducing/oxidizing conditions o f annealing which are well correlated to the superconducting propertie s. A relationship between T-c's of this Tl1.22Ba1.04Sr0.96Ca0.78Cu2O7- delta 'BaSr' 1212 crystal and Cu-Cu distances is compared with the res ults obtained for pure 'Ba' and 'Sr' 1212 phases.