Influence of synthesis conditions on phase formation and magnetic properties of Hg-based high temperature superconductors

Experiments on ampoule synthesis of high temperature superconductors Hg 1 − x Pb x Ba 2 − y Sr y Ca n − 1 Cu n O 8+ δ ( x∼0.2–0.3, 0.5⩽ y⩽1.5) were carried out under different oxygen partial pressures (PO 2) controlled by redox couples Cu 2O/CuO, CoO/Co 3O 4 and Pd/PdO at temperature range 690⩽ t s⩽850 °C. It was found that at temperatures t s>700 °C and low PO 2 the higher Hg-HTS members are more stable than the lower ones. Optimization of PO 2 allowed to prepare the sample containing about 80% of 1234 phase ( n=4) and to decrease the synthesis temperature of 1223 phase ( n=3) down to 690 °C. Investigation of magnetic properties of synthesized samples has shown that appreciable decrease of critical temperature with Sr content y is not noticeable up to y=1.0, but as y increasing as t s decreasing rises the irreversibility line H irr( T) and enhances the intragrain critical current density under applied magnetic field. On the contrary, when y>1 its increasing leads to an abrupt decrease of T c and, as a consequence, to lowering of H irr( T) line at temperatures T>100 K. Wave-like intragrain microinhomogeneities, which could serve as pinning centers, were observed by TEM.