Exploring the Verwey-Type Transition in GdBaFe2O5+w Using 57Fe Moessbauer Spectroscopy

57Fe Moessbauer spectroscopy was used to study the double perovskite GdBaFe2O5+w, which exhibits mixing of the integer valence states of iron. The valence mixing/separation process Fe2++Fe3+↔2Fe2.5+ was investigated as a function of temperature. For nearly stoichiometric compositions of w∼0, a two-step Verwey-type transition is registered that separates Fe2.5+ into intermediate valence- and spin states Fe2.5-elementof and Fe2.5+elementof and then into the integer valences Fe2+ and Fe3+. Both steps are accompanied by a decrease in electrical conductivity, altogether by two orders of magnitude. Seebeck measurements identify holes as dominating charge carriers, with activation energy for hopping of ∼0.10 eV in the valence-mixed state. It is inferred that the mixing electrons are not simply delocalized over the lattice, but rather form bridges connecting pairs of adjacent Fe atoms along the c axis.