document: The observed conductivity of A2FeMoO6–δ (A = Ca, Sr, Ba) [7] was linked to a potential double exchange mechanism, with conduction between Fe3+-O-Mo-O-Fe2+. Double-exchange mechanisms, as proposed by Zener [23], posit that electron transfer between ions in different oxidation states may be facilitated if the electron does not have to alter its spin state. Replacement of Mo with Fe in this mechanism would be expected to result in a reduction of the conductivity through reduction of the available percolation pathways, unless delocalisation of Fe electrons through Fe2+-O-Fe3+ exchange could also occur. Double exchange mechanisms have been observed previously for mixed valent iron in iron oxides [24], and, as iron is known to exist in a mixed valent state for Ca2–xSrxFeMoO6–δ [25], this provides a plausible explanation for the observed metallic conductivity. Band structure calculations and Mossbauer spectroscopy could be utilised to further elucidate the conduction mechanism for these compounds, however this is outside the scope of this enquiry.
extractive_keyphrases: a2femoo6–δ ba band structure calculations ca ca2–xsrxfemoo6–δ compounds conduction mechanism delocalisation of fe electrons double exchange mechanism double exchange mechanisms electron fe ions iron metallic conductivity mo mossbauer spectroscopy observed conductivity oxidation percolation pathways reduction of the conductivity replacement sr
abstractive_keyphrases: double-exchange mechanisms electron transfer fe2+-o-fe3+ fe2+-o-fe3+ exchange fe3+-o-mo-o-fe2+ fe electrons iron in iron oxides reduction of the available percolation pathways