Preparation, properties and Mossbauer spectra of cobalt-doped (Tl,Pb)-1223 superconductors

The introduction of the Co both as natural Co and as the Mossbauer active Co-57 into a Tl-based 1223 superconductor was investigated. The Co-doped superconductors of the composition (Tl0.5Pb0.5)(Ba0.2Sr0.8)(2)Ca-2(Cu1-xCox)(3)O-y (x: 0.01, 0.03, 0.05 and 0.10) were characterised with respect to critical temperature, critical current densities, microstructure and composition of the superconducting and secondary phases. Co doping at these low levels decreased the critical temperatures by 4 K to 113 K for x = 0.01, and by 15 K to 102 K for x = 0.03 and by 35 K to 82 K for x = 0.05. Energy dispersive X-ray analysis showed that Co was incorporated in the superconducting crystallites. The Mossbauer emission spectrum of Co-57 was resolved into a slightly asymmetric doublet, assigned to Co substituting for Cu in the square planar, fourfold oxygen co-ordinated site. (C) 2002 Published by Elsevier Science B.V.