3 research outputs found
Oxygen Release and Incorporation Behaviors in BaFeO<sub>3</sub> Polymorphs with Unusually High-Valence Fe<sup>4+</sup>
Fully
oxygenated perovskite BaFeO3 containing unusually
high-valence Fe4+ shows three crystal polymorphs with the
same chemical composition. The 3C-type BaFeO3 has a simple
cubic perovskite structure consisting of corner-sharing FeO6 octahedra, while the 6H- and 12R-type BaFeO3 have hexagonal
perovskite structures consisting of both corner-sharing and face-sharing
FeO6 octahedra. The compounds readily release oxygen into
the air to reduce the high-valence state of the Fe ions, but the oxygen
release behaviors strongly depend on the crystal structure. The 3C-type
BaFeO3 releases oxygen topotactically from the corner-shared
sites of the FeO6 octahedra at a temperature as low as
130 °C. In contrast, the 6H- and 12R-type BaFeO3 preferentially
release oxygen from the face-shared sites above 320 and 460 °C,
respectively, although they include the corner-shared sites in the
crystal structures. The resultant oxygen-deficient 3C-type BaFeO2.5 does not incorporate back oxygen in air, whereas the 12R-type
hexagonal structure shows completely reversible oxygen release and
incorporation in air. Once the 12R-type structure is established,
unusually high-valence states such as Fe4+ can be stabilized
without extreme conditions
Oxygen Release and Incorporation Behaviors in BaFeO<sub>3</sub> Polymorphs with Unusually High-Valence Fe<sup>4+</sup>
Fully
oxygenated perovskite BaFeO3 containing unusually
high-valence Fe4+ shows three crystal polymorphs with the
same chemical composition. The 3C-type BaFeO3 has a simple
cubic perovskite structure consisting of corner-sharing FeO6 octahedra, while the 6H- and 12R-type BaFeO3 have hexagonal
perovskite structures consisting of both corner-sharing and face-sharing
FeO6 octahedra. The compounds readily release oxygen into
the air to reduce the high-valence state of the Fe ions, but the oxygen
release behaviors strongly depend on the crystal structure. The 3C-type
BaFeO3 releases oxygen topotactically from the corner-shared
sites of the FeO6 octahedra at a temperature as low as
130 °C. In contrast, the 6H- and 12R-type BaFeO3 preferentially
release oxygen from the face-shared sites above 320 and 460 °C,
respectively, although they include the corner-shared sites in the
crystal structures. The resultant oxygen-deficient 3C-type BaFeO2.5 does not incorporate back oxygen in air, whereas the 12R-type
hexagonal structure shows completely reversible oxygen release and
incorporation in air. Once the 12R-type structure is established,
unusually high-valence states such as Fe4+ can be stabilized
without extreme conditions
Hexagonal Perovskite Ba<sub>4</sub>Fe<sub>3</sub>NiO<sub>12</sub> Containing Tetravalent Fe and Ni Ions
BaFe<sub><i>x</i></sub>Ni<sub>1–<i>x</i></sub>O<sub>3</sub> with end members
of BaNiO<sub>3</sub> (<i>x</i> = 0) and BaFeO<sub>3</sub> (<i>x</i> = 1), which, respectively, adopt the 2H and
6H hexagonal perovskite structures, were synthesized, and their crystal
structures were investigated. A new single phase, Ba<sub>4</sub>Fe<sub>3</sub>NiO<sub>12</sub> (<i>x</i> = 0.75), that adopts
the 12R perovskite structure with the space group <i>R</i>3Ì…<i>m</i> (<i>a</i> = 5.66564(7) Ã…
and <i>c</i> = 27.8416(3) Ã…), was found to be stabilized.
Mössbauer spectroscopy results and structure analysis using
synchrotron and neutron powder diffraction data revealed that nominal
Fe<sup>3+</sup> occupies the corner-sharing octahedral site while
the unusually high valence Fe<sup>4+</sup> and Ni<sup>4+</sup> occupy
the face-sharing octahedral sites in the trimers, giving a charge
formula of Ba<sub>4</sub>Fe<sup>3+</sup>ÂFe<sup>4+</sup><sub>2</sub>Ni<sup>4+</sup>O<sub>11.5</sub>. The magnetic properties of
the compound are also discussed