Valence mixing, separation and ordering in double-cell perovskite GdBaFe₂O_5+w

Thermally induced valence mixing Fe²⁺ + Fe³⁺ ↔ Fe^2.5+ is studied in GdBaFe₂O_5+w by ⁵⁷Fe Mössbauer spectroscopy. For compositions close to 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 proportional decrease in electrical conductivity, altogether by approximately two orders of magnitude, and small magnetoresistance peaks are observed. Seebeck measurements identify holes as charge carriers. Increasing oxygen nonstoichiometry eliminates rapidly the first step, whereas the generally stronger Verwey transition proper persists up to levels of w≈0.25.