Charge ordering and successive phase transitions of mixed-valence iron oxide GdBaFe2O5

We investigated the correlations between the crystal structures and electrical properties of the double-perovskite GdBaFe2O5 over a broad range of temperatures. We examined the successive phase transitions and determined the crystal structures of four crystallographic phases of GdBaFe2O5 using electron diffraction and synchrotron radiation X-ray powder diffraction (SRXRD). The valence states of the Fe sites in the respective phases were deduced from the SRXRD data. We observed two types of charge-ordered states caused by charge and orbital orderings at Fe ions. The electrical resistivity displayed a metal-insulator transition, which is compatible with charge-ordered states. The structural phase transitions in GdBaFe2O5 were observed to be related to the charge-, magnetic-, and orbital-ordered states.