Mossbauer spectroscopy and preparation of Hg-1223, Tl-1223 and Tl-1212 superconductors

Introduction of the Mossbauer nuclei Fe-57 into Hg-1223 phase and that of F e-57 and Eu-151 into Tl-1212 and Tl-1223 superconductors were investigated. Samples of high phase purity were obtained. Scanning electron microscopy a nd optical microscopy in normal and polarized light were employed to study the microstructure of the specimens. Energy dispersive X-ray analysis showe d that Fe-57 in the Hg-based samples and Fe-57 as well as Eu-151 in the Tl- based compounds, entered superconducting phases. Incorporation of Eu3+ into the superconducting phase favored the formation of the Tl-1212 phase. Moss bauer spectroscopy showed that Eu3+ entered the Ca-site. Two doublets found in the Fe-57 Mossbauer spectra in both the Hg-1223 and the Tl-1223 phase r eferred to two different micro-environments of Fe3+. The assignment of the Fe-57 Mossbauer spectra was made under the assumption that Fe favored the 1 223 phase. Fe3+ may replace Cu in both the square pyramidal, five-fold oxyg en coordinated Cu sites between the Ca and Ba-(Sr)-O layers, and in the squ are planar, four-fold oxygen coordinated Cu sites the Ca layers in the supe rconducting phases in both the Hg- and the Tl-based materials. From the rel ative areas of the two doublets, we concluded that the Fe3+ preferred the s quare planar Cu site.