Transfert électronique d'intervalence entre sites inéquivalents: Étude de CaFe3O5 par spectrométrie Mössbauer

As in magnetite Fe₃O₄, calcium ferrite CaFe₃O₅ is an oxide in which electron transfer occurs between the iron ions ($\text{Fe}{}^{\mathrm{3+}}\text{Fe}{}^{\mathrm{2+}}\text{= 2}$). This intervalence exchange process has been studied by ⁵⁷Fe Mössbauer spectroscopy and by electrical conductivity measurements. In CaFe₃O₅, the Fe³⁺ and Fe²⁺ ions occupy different crystallographic sites and have a deformed octahedral coordination. Each Fe²⁺O₆ octahedron shares an edge with two Fe³⁺O₆ octahedra. In the antiferromagnetic region (T_N = 282 ± 2 K), the Fe³⁺ and Fe²⁺ ions are well differentiated. Thermally-activated electron transfer is observed above t_n, in the paramagnetic region and is well characterized by the Mössbauer spectra. These are analyzed using the hypothesis of an electron jump limited to a trimer Fe³⁺Fe²⁺Fe³⁺ which leads to a relaxation time of 180 ns at 298 K and 80 ns at 400 K. Within this temperature interval, the process follows the Arrhenius law with an activation energy of 0.10 eV. Electrical conductivity measurements lead to similar results with an activation energy of 0.09 ± 0.02 eV.