Effect of nonstoichiometry on crystal structure, charge and spin states of cobalt ions in Tb1−yBa1+yCo2−xO5.5−δ: Neutron diffraction and soft X-ray absorption spectroscopy studies

Abstract

Neutron diffraction and soft X-ray absorption spectroscopy have been applied for studying crystal structure, charges, and spin states of cobalt ions in nonstoichiometric Tb1−yBa1+yCo2−xO5.5−δ cobaltites. The following single-phase samples with defects in anion and cation sublattices were synthesized and investigated: TbBaCo2O5.38, TbBaCo1.95O5.31, TbBaCo1.90O5.24, and Tb0.95Ba1.05Co1.90O5.21. Co and Tb charge states have been estimated from soft X-ray absorption spectra. The anisotropic nature of changes of lattice parameters depending on the content of cobalt and oxygen vacancies has been established. A correlation between cobalt and oxygen vacancies in double layered cobaltites and a role of apical and planar oxygen in the formation of the Tb1−yBa1+yCo2−xO5.5−δ structure have been shown. Oxygen vacancies are found to be unevenly distributed across the unit cell. Most vacancies are formed in the O (4u) and O (2s) planar positions. The O (1g) apical positions are also partly vacant. The high spin state of Co3+ ions in octahedra has been found to be realized. This conclusion obtained from neutron diffraction data is consistent with the results of X-ray absorption spectroscopy.