Synthesis and Physical Properties of the Hg-Based Superconductors with the 1212 Structure: Hg_0.7 V_0.3Sr₂Ca_1-xY_xCu₂O_6+gd

Abstract

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_0.7V_0.3Sr₂Ca{i−x}Y _x Cu₂O_6+δ. The lattice parameters show an increase in thea axis and a significant decrease in thec axis with increase inx. A maximumT _c of 85K 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 thec 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 presenceof defects in the proximity of the conduction plane in the high-T _c copper oxide system has an adverse effect on the superconducting properties.