Moessbauer investigation of 57Fe doped GdBa2Cu3O7-y

We report on Moessbauer spectroscopy of GdBa2(Cu1-x57Fex)3O7-x superconductors with iron contents x=0.005, 0.01 and 0.02. The critical temperatures were determined to be Tc=78, 65 and 56 K respectively for increasing x. Moessbauer spectra were recorded at room temperature and at 5 K. No magnetic ordering appears. All spectra can be analyzed using three quadrupole split doublets whose relative intensities and hyperfine parameters vary with x and with temperature. From the intensities we conclude that roughly 90% of the Fe substitutes for Cu(1) and the rest for Cu(2). (orig.)