Exploring the Verwey-type transition in GdBaFe₂O_5+w using ⁵⁷Fe Mössbauer spectroscopy

⁵⁷Fe Mössbauer spectroscopy was used to study the double perovskite GdBaFe₂O_5+w, which exhibits mixing of the integer valence states of iron. The valence mixing/separation process Fe²⁺ + Fe³⁺ ↔ 2Fe^2.5+ was investigated as a function of temperature. For nearly stoichiometric compositions of w ≈ 0, a two-step Verwey-type transition is registered that separates Fe^2.5+ into intermediate valence- and spin states Fe^2.5-ε and Fe^2.5+ε 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.