ELECTRON DELOCALIZATION ON IRON SITES IN THALLIUM AND LEAD-BASED FERRITES (TL,PB)SR3FE2O8 AND (TL,PB)SR4FE2O9 - MOSSBAUER STUDY

A Mossbauer study on powder samples of the two series of iron based ox ides ASr(3)Fe(2)O(8) and ASr(4)Fe(2)O(9) where A is a 5d(10) cation (A = Tl3+, Pb4+, Hg2+) is reported. In both families, the thermal evolut ion of the Mossbauer spectra associated with susceptibility measuremen ts provides evidence for a mixture of high-spin localized Fe3+ and Fe4 + at low temperature and of different mixed valence states at higher t emperatures, which are interpreted in terms of a dynamical electron de localization. The number of intermediate-valence iron sites observed b y Mossbauer spectroscopy depends on the magnitude of the hopping frequ ency with respect to the characteristic Fe-57 Mossbauer frequency. Thi s process, governed by both the temperature and the mean charge of the A cations, is also demonstrated by the electronic conductivity measur ements. Because of the low dimensionality of the structure, O-9 compou nds allow the electron delocalization to start at lower temperatures t han in the O-8 family.