CHARACTERIZATION OF THE MIXED-VALENT TL-1222 FERRITE TLSR4FE2O9-DELTA

The mixed valent Tl-1222 ferrite TlSr4Fe2O9 has been prepared and char acterized using various techniques. Its structural features as well as chemical analyses evidenced that this compound shows some non-stoichi ometry and that it can be formulated Tl0.96Sr4Fe2O8.70. Magnetic measu rements as well as Mossbauer spectroscopy data agree well with this fo rmulation and have been interpreted on the basis of a temperature depe ndent mixture of trivalent iron and of a mixed valent iron; it results from a fast electron hopping from site to site whose number increases with temperature even though a complete delocalization is not reached . On account of the Mossbauer results, a charge transfer from oxygen t o iron has been assumed at increasing temperature. These peculiar elec tronic configurations of iron cations result from the presence of thal lium cations in a 2D-type material.