Large Oxygen Nonstoichiometry
in La<sub>0.77</sub>Sr<sub>3.23</sub>Co<sub>2.75</sub>C<sub>0.25</sub>O<sub>8.40+δ</sub> Oxide (δ = 0, 1.3) Related to <i>n</i> = 3 RP Series
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Abstract
An original Ruddlesden–Popper phase, La<sub>0.77</sub>Sr<sub>3.23</sub>Co<sub>2.75</sub>C<sub>0.25</sub>O<sub>8.40+δ</sub>, was isolated and studied by electron, X-ray, and neutron diffraction.
This structure has complex crystal chemistry resulting from a high
degree of flexibility in the structure, comprising the disordered
introduction of carbonates into a cobalt layer and an important oxygen
deficiency with a preferential repartition of vacancies along the
layers stacking sequence. The former is necessary for the stabilization
of the system, while the latter can be tuned by postsynthetic treatment,
yielding in a large variety of cobalt species formal oxidation states
ranging from Co<sup>2+</sup>/Co<sup>3+</sup> in the as-made phase
to Co<sup>3+</sup>/Co<sup>4+</sup> when annealed under oxygen pressure.
The potential richness deriving from this flexibility is illustrated
in terms of the magnetotransport properties and includes a resistivity
that varies within a range of 5 orders of magnitude after modulation
of the oxygen content with the appearance of negative magnetoresistance
and ferromagnetic interactions due to Co<sup>3+</sup>/Co<sup>4+</sup> mixed-valence state
