A NEW ELABORATION PROCESS OF THE SUPERCONDUCTING TL2BA2CU1O6 PHASE WITH TC=90K

We have synthesized high quality ceramic Tl2Ba2Cu1O6+delta (2201) samples using the high pressure, high temperature route. At high oxygen content, the structure is orthorhombic and the samples are metallic but non-superconducting. Upon lowering the oxygen content, the symmetry changes from orthorhombic to tetragonal or pseudo-tetragonal. In the latter phase, the maximum superconducting critical temperature reaches 92 K. Optical micrographs show large 2201 grains and some traces of impurity phases like Tl2Ba2O5, Ba2Cu3Ox and CuO. X-ray diffraction shows only the 2201 phase. Plasma emission spectroscopy indicates that the global sample stoichiometry is Tl:Ba:Cu=2:2:1. This analysis proves that the high pressure route effectively prevents the thallium evaporation. HREM investigations exclude the possibility of cation vacancies. Microprobe analyses (EDAX) show no variation of the cation stoichiometry between the 2201 grains. X-ray diffraction on a superconducting single crystal yields a refined composition Tl1.94Ba2Cu1.06O6.