DISTRIBUTION OF IRON OVER COPPER SETS IN GDBA2(CU1-XFEX)3O7-DELTA

Iron-doped compounds GdBa2(Cu1-xFex) 3O7-δ with 0.03≤x≤0.25 and δ<0.1 as well as with δ>0.7 have been investigated by Mössbauer spectroscopy with 155Gd and 5 7Fe. For each iron concentration x, samples with different transition temperatures Tc (at constant oxygen content, δ<0.1) were prepared and investigated. Samples were reduced to an oxygen deficiency δ>0.7 either at 650°C under vacuum or at 950°C in a N 2 atmosphere, followed by a quench to room temperature. A comparison of 57Fe Mössbauer spectra for samples treated by these different procedures has yielded an unambiguous determination of the fraction of iron atoms occupying Cu(2) sites. This fraction extrapolates to ∼4% for x→0. It increases monotonously with x to ∼17% for x=0.12. In samples reduced at 950°C for 44 h, the fraction of iron on Cu(2) sites is doubled, and this large Cu(2) site occupancy is maintained when the material is reoxidized at 450°C. From the systematic variation of the parameters associated with the remaining quadrupole doublets with iron concentration x and with Tc (for constant x), tentative assignments of the doublets to specific oxygen coordinations of Fe on Cu(1) sites were derived. For 155Gd, both the quadrupole splitting and the isomer shift vary smoothly with iron concentration. No influence of an extra charge associated with iron on Cu(2) sites in the vicinity of Gd nuclei is discernible.