Synthesis and physical properties of the Hg-based superconductors with the 1212 structure:: Hg0.7V0.3Sr2Ca1-xYxCu2O6+δ

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 greater than or equal to 0.4 in the system Hg(0.7)V(0.3)Sr(2)Ca1(-x)Y(x)CU(2)O(6+delta). 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 magnetization-hysteresis 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 Y3+ in the Ca2+ layer adjacent to the CuO2 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.