Correlation between the metal-insulator transition and the electronic density of states near the Fermi level in oxygen-deficient perovskite-type NdBaCo2O5.5

X-ray absorption spectra near the absorption edges of Nd, Ba, and Co ions have been taken to study the metal-insulator (M-I) transition occurring at about 360 K (T-MI) in NdBaCO2O5.5. In the metallic and insulating phases, the electronic density of states (DOS) just above the Fermi level (E,) consists of the high-density unoccupied bands of Nd-5d sigma and Ba-5d sigma. The narrow charge-transfer Co-3d(7)(e(g))(L) under bar band and the broad charge-transfer Co-3d(t(2g))(n)(L) under bar (m) bands occupy positions just below and near E-F, respectively, where (L) under bar represent a hole in the O-2p valence band. The negative charge-transfer Nd-4d(n)(L) under bar'(m) and Ba4d(n)(L) under bar'(m) hybridized bands are also just below E-F, where (L) under bar' represent a hole in the fully occupied 4d band of the Nd and Ba ions. It is suggested that the metallic conductivity above T-MI arises from the itinerant (p-d) hybridized holes having an 3d(e(g))-symmetry in the top of the O-2p valence band. The structural deformation localized around the Ba ions but not the Nd and Co ions affects the crystal-field splitting of the Co3d orbitals in the CoO polyhedra and the hybridization between the outer orbitals of the cations (Nd, Ba, Co ions) and the O-2p orbitals.