INVESTIGATIONS ON T1_2Ba_2CuO_(6+#delta#) AND T1_(0.5)Pb_(0.5)Sr_2Ca_(1-x)Y_xCu_2O_7 IN RELATION TO OPTIMISATION OF TRANSITION TEMPERATURES (T_#chi#s) AND STRUCTURAL-MICROSTRUCTURAL CHARACTERISTICS

摘要

Two modes of carrier density reduction for 'T_C ' optimisation - the reduction through annealing (approx 300 deg C) in hydrogenated argon for T1_2Ba_2CuO_(6+#delta#) (2201) and the cationic substitution forming the T1_(0.5)Pb_(0.5)Sr_2Ca_(1-x)Y_xCu_2O_7 (1212) phase, have been carried out. It has been found that T_c in 2201 phase gets enhanced from < 10K to approx 75K on reduction through annealing in hydrogenated argon and in 1212 phase from 80 to approx 90K on reduction through cationic substitution Y~(3+) -> Ca~(2+) by forming T1_(0.5)Pb_(0.5)Sr_2Ca_(1-x)Y_xCu_2O_7 (1212). TEM explorations of the reduced 2201 phase have revealed the presence of superlattice structure nucleated presumably due to creation and ordering of oxygen vacancies leading to reduction of carrier (hole) density. However, the substitution Y~(3+) for Ca~(2+) in T1_(0.5)Pb_(0.5)Sr_2Ca_(0.8)Y_(0.2)Cu_2O_y, does not lead to any noticeable structural disordering effect implying the formation of a simple substitutional compound which would have reduced hole density through partial replacement of appropriate ions (Y~(3+) -> Ca~(2+)).