Complex susceptibilities of Co-substituted YBa₂Cu₃O_7-d synthesized by the polymerized complex method

Practically impurity-free ceramic samples of YBa₂Cu_3-xCo_xO_7-d with x = 0-0.5 and Y_0.8Ca_0.2Ba₂Cu_2.8Co_0.2O_7-d were synthesized by the polymerized complex method through two different routes, starting either from metal nitrates (route P) or from metal carbonates (route Q). Better compositional homogeneity was achieved in the latter route, where neither colloids nor precipitates were formed during the polymerization procedure. The magnetic response of the samples was measured in terms of AC magnetic susceptibilities and variation of the onset temperature T_c(onset) of the superconducting transition was precisely determined as a function of Co content. The T_c(onset) were almost unaffected for 0 ≤ x ≤ 0.5, decreased linearly with ΔT_c/Δx ∼ -220 K for 0.07 ≤ x ≤ 0.3, and the superconductivity vanished for x ≥ 0.4. The depression of T_c was interpreted in terms of the localization of holes in the Cu(1)O_y planes. The replacement of 20% of Y with Ca in YBa₂Cu_2.8Co_0.2O_7-d enhanced T_c from the initial value of 57.1 K up to 88.6 K. It was suggested that the rise in T_c induced by Ca doping was connected with the delocalization effect of holes in which the hole charge transfer from the Cu(1)O_y planes to the Cu(2)O₂ conduction sheets played a crucial role.