Exploring the Verwey-Type Transition in GdBaFe_2O_(5+ω) Using ~(57)Fe Moerssbauer Spectroscopy

摘要

~(57)Fe Moessbauer spectroscopy was used to study the double perovskite GdBaFe_2O_(5+ω), which exhibits mixing of the integer valence states of iron. The valence mixing/separation process Fe~(2+) + Fe~(3+) ←→ 2Fe~(2.5+) was investigated as a function of temperature. For nearly stoichiometric compositions of ω ≈ 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 Fe~(2+) and Fe~(3+). 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.