Substitution effects of Fe and Co for Cu in YBa2(Cu1-xMx)4O8 superconductors

The effects of substitutions of Fe and Co for Cu in YBa2(Cu1-xMx)4O8 (124 phase, M=Fe and Co) were investigated. The single phase of the 124 structure was synthesized at 1040degC in an oxygen partial pressure of 40 MPa in the region of 0<x≤0.02. When x exceeded 0.02, the formation of both YBa2Cu3O7-y (123 phase) and Y2Ba4Cu7O15-y (247 phase) structures occurred in cases such as Fe and Co substitutions. From the fact that excess substitution suppresses the formation of the 124 phase, i.e. the double Cu-O chains, it is suggested that Fe and Co occupy the Cu(1), Cu-O chain, site. In the single-phase region of the 124 phase, the a and b lattice parameters increase slightly, while the c lattice parameter shows little decrease. With increasing dopant contents Tc decreased at a rate of 20 K/at.%, which was larger than the case of the 123 phase. The large depression of Tc possibly originates from the deference of the Cu-O chain structure. (orig.)