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Atomic and Local Electronic Structures of Ca2AlMnO5+δ as an Oxygen Storage Material
journal contribution
posted on 2016-12-08, 00:00 authored by Genki Saito, Yuji Kunisada, Kazuki Hayami, Takahiro Nomura, Norihito SakaguchiWe
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.
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Keywords
oxygen storageHigh-angle annular dark-field scanning transmission electron microscopyMn L-edge electron energy-loss near-edge structure2 AlMnO 5AlO 4 tetrahedraMnO 6 octahedral sitesoxygen storage abilitysite-resolved oxygen K-ELNESform octahedral AlO 6Local Electronic StructuresOxygen Storage Materialoxygen storage materialMn oxidation stateAlO 6 octahedra
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