Electronic States of Cobalt Ions in EuBaCo2O5 + δ Layered Cobaltites

The oxygen K -edge X-ray absorption spectra in EuBaCo 2 O 5.52 ± 0.02 and EuBaCo 2 O 5.24 ± 0.02 cobaltites are measured at temperatures of 300 and 440 K, which are below and above the metal–insulator transition temperature, respectively. According to these spectra, the substitution of Co 2+ ions for some Co 3+ ions with a decrease in the oxygen content in the chemical formula of a cobaltite and, hence, an increase in the relative fraction of CoO 5 pyramids with respect to the number of CoO 6 octahedra leads to an increase in the band gap by about 0.3 eV. The band structure of EuBaCo 2 O 5.5 is calculated using the method of linearized muffin-tin orbitals in the local density approximation taking into account the local Coulomb interaction. It is found that the low-spin state of Co 3+ ions occurs in CoO 6 octahedra in EuBaCo 2 O 5.5 , whereas the high-spin state of Co 3+ ions is typical of cobalt ions in pyramids.