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

Experiments on ampoule synthesis of high temperature superconductors Hg1-xPbxBa2-ySryCan-1CunO8+δ (x∼0.2-0.3, 0.5≤y≤1.5) were carried out under different oxygen partial pressures (PO2) controlled by redox couples Cu2O/CuO, CoO/Co3O4 and Pd/PdO at temperature range 690≤ts≤850 deg. C. It was found that at temperatures ts>700 deg. C and low PO2 the higher Hg-HTS members are more stable than the lower ones. Optimization of PO2 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 deg. 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 ts decreasing rises the irreversibility line Hirr(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 Tc and, as a consequence, to lowering of Hirr(T) line at temperatures T>100 K. Wave-like intragrain microinhomogeneities, which could serve as pinning centers, were observed by TEM