COMPLEX SUSCEPTIBILITIES OF CO-SUBSTITUTED YBA2CU3O7-D SYNTHESIZED BYTHE POLYMERIZED COMPLEX METHOD

Practically impurity-free ceramic samples of YBa2Cu3-xCoxO7-d with x = 0-0.5 and Y0.8Ca0.2Ba2Cu2.8Co0.2O7-d were synthesized by the polymeri zed complex method through two different routes, starting either from metal nitrates (route P) or from metal carbonates (route Q) Better com positional homogeneity was achieved in the latter route, where neither colloids nor precipitates were formed during the polymerization proce dure. 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 l ess than or equal to x less than or equal to 0.05, decreased linearly with Delta T-c/Delta x similar to -220 K for 0.07 less than or equal t o x less than or equal to 0.3, and the superconductivity vanished for x greater than or equal to 0.4. The depression of T-c was interpreted in terms of the localization of holes in the Cu(1)O-y planes. The repl acement of 20% of Y with Ca in YBa2Cu2.8Co0.2O7-d enhanced T-c from th e 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 eff ect of holes in which the hole charge transfer from the Cu(1)O-y plane s to the Cu(2)O-2 conduction sheets played a crucial role.