A new elaboration process of the superconducting Tl2Ba2Cu1O6 phase with Tc=90K

International audienceWe have synthesized high quality ceramic Tl2Ba2Cu1O6+δ (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

Characterization of the 105 K superconductor Tl₂Ba₂CaCu₂O₈ (“2212”)

Homogeneous Tl1.8Ba2CaCu2O8 ceramics were synthesized by a novel route, starting from Tl2Ba2O5 precursors and using high gas pressures. This method allows tight control of thallium losses, resulting in dense, large-grained samples with sharp superconducting transitions above 105 K. Results of a characterization by X-ray diffraction, optical micrographs and micro-probe analysis are presented, together with selected physical properties

Equilibrium diagram T_c(T;p(O₂)) of Tl₂Ba₂CuO₆

Homogeneous Tl₂Ba₂CuOₓ ceramics were synthesized by a novel route allowing tight control of thallium losses, starting from Tl₂Ba₂O₅ precursors and using high gas pressures. Annealing followed by fast quenching into gallium allowed us to map the critical temperature (0 to 92 K) and the crystal symmetry (orthorhombic or tetragonal) in the (T;p(O₂)) plane. The orthorhombic phase is found to be stoichiometric, whereas the tetragonal phase has a homogeneity domain in the thallium concentration. The critical temperature, determined by the oxygen concentration, is not sensitive to the crystal structure. Selected physical properties of Tl₂Ba₂CuOₓ compounds are presented

Phase diagram of the Tl₂Ba₂CuO₆ compounds in the T, <i>p<i>(O₂) plane

We present a detailed investigation of the structural and superconducting properties of Tl2Ba2CuOx compounds as a function of oxygen pressure and annealing temperature prior to quenching. The crystal structure depends both on the oxygen concentration and on thallium deficiency. At the stoichiometric cation composition Tl:Ba:Cu=2:2:1, the parameters p(O2) and Tquench uniquely define the structure and the superconducting Tc. With adequate heat treatment sequences, reversible transitions from the tetragonal to the orthorhombic phase have been observed

Thermodynamic and kinetic studies of the phase transitions in Tl₂Ba₂CuO_6±x

We present a kinetic and thermodynamic analysis of the phase transitions in stoichiometric Tl2Ba2CuO6 samples prepared under a pressure of 100 bar. We have found a first-order transformation which maintains the orthorhombic structure from high pressure to 1 bar. The as-prepared tetragonal structure in an inert gas converts to orthorhombic symmetry when annealed in an oxygen flux. The activation energies have been deduced from a dynamic analysis of weight losses during the phase changes. The enthalpy and entropy for the formation of oxygen vacancies have been determined

Magnetic properties of the Tl₂Ba₂Cu₁O_6+δ 90K superconductor

We have measured magnetic properties of high quality Tl2Ba2Cu1O6+δ 90 K tetragonal superconducting and orthorhombic metallic non-superconducting ceramics. The AC susceptibility shows distinct intra- and intergrain transitions down to H=0.004 Oe rms. The field cooling measurement at 20 Oe shows a sharp transition at 90 K and 50% of full flux exclusion at 5 K. At T>150 K, i.e. above the region where superconducting fluctuations contribute, the normal-state susceptibility is temperature independent and diamagnetic with a value of −5×10−8 emu/g. The intergrain critical current densities determined at B=1 T, using Bean's critical state model, are very low, 6×103 A/cm2 at 5 K and below 1 A/cm2 at 40 K