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

We have synthesized high quality ceramic Tl2Ba2Cu1O6+delta (2201) samp les using the high pressure, high temperature route. At high oxygen co ntent, the structure is orthorhombic and the samples are metallic but non-superconducting. Upon lowering the oxygen content, the symmetry ch anges from orthorhombic to tetragonal or pseudo-tetragonal. In the lat ter phase, the maximum superconducting critical temperature reaches 92 K. Optical micrographs show large 2201 grains and some traces of impu rity phases like Tl2Ba2O5, Ba2Cu3Ox and CuO. X-ray diffraction shows o nly the 2201 phase. Plasma emission spectroscopy indicates that the gl obal 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. Mic roprobe 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.