Neutron diffraction and Mossbauer spectroscopy studies of the mixed valent Tl-1212 ferrite (Tl0.5Pb0.5)Sr3Fe2O8

The mixed valent 1212-ferrite (Tl0.5Pb0.5)Sr3Fe2O8 has been characterized using powder neutron diffraction data collected at 1.5 and 293 K. The structure is confirmed to be close to that of the Tl-1212 superconductor (Tl0.5Pb0.5)Sr2CaCu2O7 but with no deviation from a full occupancy of (Tl, Pb) and oxygen sites. Fe-57-Mossbauer spectroscopy data depend upon the temperature between 4.2 and 293 K. They have been explained on the basis of a mixture of trivalent and tetravalent iron states at 4.2 and 150 K. At temperature higher than 150 K the existence of intermediate iron states Fe-n+ (3 < n < 4) due to electron transfer between Fe3+ and Fe4+ sites has to be taken into account to fit the data. EFG calculations based upon neutron diffraction results at 293 K confirm that local displacements of the apical oxygen O (2) can be considered to explain the appearance of three Mossbauer iron sites instead of the structure expected one.