Atomic and Local Electronic Structures of Ca2AlMnO5+δ as an Oxygen Storage Material

We investigated the atomic and local electronic structures of Ca2AlMnO5+δ to assess its potential as an oxygen storage material. High-angle annular dark-field scanning transmission electron microscopy was used to investigate structural changes in the material during oxygen storage. We found that the AlO4 tetrahedra convert to AlO6 octahedra during such a process. According to the Mn L-edge electron energy-loss near-edge structure (ELNES) measurements, the Mn oxidation state increased from +3 to +4 on oxygen storage. The observed site-resolved oxygen K-ELNES and first-principles electronic structure calculations showed that each nonequivalent oxygen site has different characteristics, corresponding to local chemical bonding and oxygen intake and release. For Ca2AlMnO5, the prepeak intensity was higher at MnO6 octahedral sites, indicating covalent bonding between the oxygen and Mn atoms. After oxygen storage, the ELNES spectra revealed that the Jahn–Teller distortion of the Mn sites was suppressed by the increase in the Mn oxidation state; furthermore, the spectra indicate that Mn octahedron shrank in the z-direction, accompanied by an increase in Mn–O covalent bonding, thus providing sufficient space to form octahedral AlO6. Consequently, we found that the reversible oxygen storage ability is related to the canceling of the volume changes of the Mn and Al octahedra. The electrons in Mn 3d orbitals play an important role in this structural change.