Large Oxygen Nonstoichiometry in La_0.77Sr_3.23Co_2.75C_0.25O_8.40+δ Oxide (δ = 0, 1.3) Related to <i>n</i> = 3 RP Series

An original Ruddlesden–Popper phase, La_0.77Sr_3.23Co_2.75C_0.25O_8.40+δ, 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 Co²⁺/Co³⁺ in the as-made phase to Co³⁺/Co⁴⁺ 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 Co³⁺/Co⁴⁺ mixed-valence state