⁵⁷Fe, ¹¹⁹Sn and ¹⁵¹Eu Moessbauer studies of (Tl,Pb)(Ba _0.2Sr _0.8) ₂Ca ₂Cu ₃O _y superconductors

The introduction of the Mössbauer nuclei 57Fe, 119Sn and 151Eu into Tl-based 12(n-1)n superconductors was investigated. The doped Tl-1223 and Tl-1212 superconductors were characterized with respect to critical temperatures, critical currents, phase composition, microstructure, composition of the superconducting and secondary phases and analyzed by Mössbauer spectroscopy. Energy dispersive X-ray (EDX) analysis showed that both 57Fe and 151Eu 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 electronic field gradient derived from the 151Eu spectra were consistent with Eu3+ at the Ca site. The 57Fe Mössbauer spectra refer to two different micro-environments for the Fe3+. They were assigned to Fe3+ substituting Cu between the two Ca layers and between the Ca and Ba-Sr-O layers.