QUASI-REVERSIBLE TRANSFORMATIONS AND THERMAL-STABILITY OF BIPBSR2-XCAXFEO6+Z WITH X=0 AND 1

A Mossbauer spectroscopy study of the products of the quasi-reversible reaction Fe-2201 phases reversible arrow Fe-2212 phases was performed . Differences in the values of the hyperfine field at 4.2 K and drasti c changes of the Neel temperatures offered evidence for deviations in the Bi:Pb:Sr:Fe ratios of the Fe-2212 phases obtained during 30 h anne aling at 700 degrees C from the as-prepared Bi2PbSr2Fe2O9+z and Bi2PbS rCaFe2O9+z compounds. Only slight differences in Neel temperature and hyperfine held parameters were found by comparing the as-prepared BiPb Sr2FeO6+z and BiPbSrCaFeO6+z with the Fe-2201 products obtained after a subsequent annealing for 24 h at 850 degrees C. In all Fe-2201 compo unds, besides the majority of Fe present in the form of Fe3+ with a hy perfine field of 50-51 T at 4.2 K, about 11-12% of Fe nuclei exhibited a hyperfine field of 19-20 T, which could be attributed to the valenc y state Fe4+. From the analysis of the values of the quadrupole splitt ing at 4.2 K and above the Neel temperature a spin arrangement paralle l to the c-axis for the Fe-2201 phases but in the basis plane for the Fe-2212 ones could be inferred.