Synthesis and Physical Properties of the Hg-Based Superconductors with the 1212 Structure: Hg0.7 V0.3Sr2Ca1-xYxCu2O6+gd

A partial substitution of vanadium at the Hg site helps to stabilize the Sr analogue of the Hg-1212 phase. Simultaneous replacement of Ca by a rare earth ion is found necessary to form single-phase materials. Crystalline and nearly single-phase compounds were synthesized for x≥0.4 in the system Hg 0.7 V 0.3 Sr 2 Ca{i− x }Y x Cu 2 O 6+δ . The lattice parameters show an increase in the a axis and a significant decrease in the c axis with increase in x. A maximum T c of 85 K was obtained for x= 0.4. T c decreases with increase in Y concentration. However, all the compositions in the single-phase region show a broad superconducting transition. Annealing in vacuum and re-annealing in oxygen helps to improve the diamagnetic volume fraction, but the nature of the transition remained unaffected. Studies on the magnetizationhysteresis loops suggest a poor flux pinning nature of the compounds. Although the compounds have a large decrease in anisotropy along the c axis, partial occupancy of Y 3+ in the Ca 2+ layer adjacent to the CuO 2 conduction plane is detrimental to the flux pinning behavior. This study suggests that the presence of defects in the proximity of the conduction plane in the high- T c copper oxide system has an adverse effect on the superconducting properties.