A SR-RICH 1223 CUPRATE, TL1+XBA2 3SR4/3CA2-XCU3O9, WITH A T(C) OF 110K - A SINGLE-CRYSTAL STUDY/

Single crystals of the Sr-rich 1223 thallium cuprate Tl1+xBa2/3Sr4/3Ca 2-xCu3O9 have been grown for the first time. The structure of this pha se was determined from a single-crystal X-ray diffraction study in the space group P4/mmm with a = 3.8274(4) angstrom, c = 15.524(3) angstro m. The atomic positions are similar to those found in the pure barium 1223 cuprates. Nevertheless, the present cuprate differs from the pure barium phase by the much smaller distance between two adjacent O(2) a nd O(3) planes. Consequently, the CuO5 pyramids are significantly flat tened with respect to TlBa2Ca2Cu3O9, with Cu-O apical distances of 2.4 7 angstrom for this (Ba, Sr) cuprate instead of 2.73 angstrom for the pure Ba cuprate. The susceptibility measurements performed on a single crystal show that T(c) can be increased from 90 K for the as-synthesi zed crystal to 110 K by Ar/H-2 annealing at 300-degrees-C; moreover, t his effect is reversible: by oxygen annealing, T(c) decreases again to 90 K. Comparison with the pure barium 1223 cuprate that exhibits T(c) values ranging from 110 K to 122 K demonstrates that the latter are n ot correlated with the Cu-O apical distance of the CuO5 pyramids.