Spin-state order/disorder and metal-insulator transition in GdBaCo2O5.5 : experimental determination of the underlying electronic structure

We have investigated the electronic structure of GdBaCo2O5.5 across the metal–insulator transition (MIT) using soft x-ray absorption and photoelectron spectroscopy. For the low-temperature insulating phase, we find that half of the Co3+ ions at the octahedral sites are in the low spin (LS) and the other half in the high spin (HS) state, while the Co3+ ions at the pyramidal sites are in the HS configuration. Upon increasing the temperature across the MIT, part of the LS octahedral Co3+ undergoes a spin-state transition into the HS configuration. We infer that this destroys the spin-state ordering and thus explains the decrease in resistivity. We observed that the band gap is reduced but not closed in the high-temperature phase.