Composition and structure refinement of superconducting crystals Y1-x Tb x Ba2Cu3O6+δ (x=0.10; δ=0.75)

Crystals of Y0.90Tb0.10Ba2Cu3O6.75 have been prepared by spontaneous crystallization from slowly cooled non-stoichiometric melt of the system Y-Tb-Ba-Cu-O. Average size of platelet crystals having mirror surface is 2x2, the largest - 8x9 mm with thickness 0.1-0.2 mm. The crystals have been characterized by powder X-ray diffraction and electron microprobe analysis. Tetragonal symmetry of the crystals has been determined by X-ray diffraction. Magnetic susceptibility measurements have revealed that the crystals manifest transition to superconducting state without additional annealing (T (c) = 60 K). Structures and compositions - Y/Tb ratio (sigma = 0.01) and oxygen content (sigma = 0.04) - have been refined for two single crystals. Possibility of rhombic distortion of the tetragonal symmetry is discussed.