super(57)Fe, super(119)Sn and super(151)Eu Moessbauer studies of (Tl,Pb)(Ba sub(0.2)Sr sub(0.8)) sub(2)Ca sub(2)Cu sub(3)O sub(y) superconductors

摘要

The introduction of the Moessbauer nuclei super(57)Fe, super(119)Sn and super(151)Eu into Tl-based 12(n-1)n superconductors was investigated. The doped Tl-based superconductors were characterized with respect to critical temperatures, critical currents, microstructure and composition of the superconducting and secondary phases. The samples were then analyzed by Moessbauer spectroscopy. Energy dispersive X-ray (EDX) analysis showed that both super(57)Fe and super(151)Eu were incorporated in the superconducting crystallites. Sn was found in secondary phases. Incorporation of Eu into the superconducting phase favored the formation of the Tl-1212 phase. The isomer shift and the electric field gradient derived from the super(151)Eu spectra were consistent with Eu super(3+) at the Ca site. The super(57)Fe Moessbauer spectra refer to two different micro-environments for the Fe super(3+). They were assigned to Fe super(3+) substituting Cu between the two Ca layers and between the Ca and Ba-Sr-O layers.