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 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 Tc(onset) of the superconducting transition was precisely determined as a function of Co content. The Tc(onset) were almost unaffected for 0 ≤ x ≤ 0.5, decreased linearly with ΔTc/Δx ∼ -220 K for 0.07 ≤ x ≤ 0.3, and the superconductivity vanished for x ≥ 0.4. The depression of Tc was interpreted in terms of the localization of holes in the Cu(1)Oy planes. The replacement of 20% of Y with Ca in YBa2Cu2.8Co0.2O7-d enhanced Tc from the initial value of 57.1 K up to 88.6 K. It was suggested that the rise in Tc induced by Ca doping was connected with the delocalization effect of holes in which the hole charge transfer from the Cu(1)Oy planes to the Cu(2)O2 conduction sheets played a crucial role.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Energy Engineering and Power Technology
- Electrical and Electronic Engineering