Effect of Co Substitution on the Crystal and Magnetic Structure of SrFeO_2.75−δ: Stabilization of the “314-Type” Oxygen Vacancy Ordered Structure without A-Site Ordering

Abstract

A study of the structure-composition-properties correlation is reported for the oxygen-deficient SrFeCoO (x = 0.1-0.85) materials. The introduction of Co in the parent SrFeO (SrFeO) structure revealed an interesting structural transformation. At room temperature (RT), an orthorhombic (space group Cmmm, 2√2a × 2a × √2a type, a = lattice parameter of the cubic perovskite) → tetragonal (space group P4/mmm, a × a × 2a type) → tetragonal (space group I4/mmm, 2a × 2a × 4a type) structural transformation is observed in parallel with increasing Co content and decreasing oxygen content in the structure. At the same time, a rich variation in the magnetic properties is explored. The samples with x = 0.25, 0.3 show temperature-induced magnetization reversal. With increasing Co content in the structure, magnetic interactions start to weaken due to the random distribution of Fe and Co in the structure; the x = 0.5 sample shows frustration in the magnetic behavior with much smaller magnetization value. With a further increase in the Co content in the structure, RT ferrimagnetic-type behavior is observed for the sample with x = 0.85. The nuclear and magnetic structure refinements using RT and low-temperature neutron powder diffraction (NPD, 10 K) patterns confirm the formation of a "314-type" novel oxygen vacancy ordered phase for the sample with x = 0.85, which is the first case of "314-type" novel oxygen vacancy ordering without A-site (ABO type perovskite) ordering. The magnetic structure is G-type antiferromagnetic starting at room temperature. Further, the stabilization of the "314-type" complex superstructure is related to the ordering of oxygen vacancies in the oxygen-deficient Co-O layers, and the same assists in building a network of Co ions with different coordination environments, each with different spin states, and forms the spin-state ordering.

Moderate magnetic field induced large exchange bias effect in ferrimagnetic 314—Sr3YCo4O10.5material

“…The RT XRD pattern indicates that the compound can be isolated as a single phase, crystallizing in a tetragonal symmetry with I4/mmm space group (S.G.). As suggested previously [17,18,19], the present structure consists of alternating oxygen-replete and oxygen deficient layers [ Fig.1] in close similarity to the Brownmillerite structure [20]. Transmission electron micrograph shows that the particles possess a quasi-spherical morphology with an average size of 0.3 µm.…”

SrCo0.85Fe0.15O2.62 – Oxygen Deficient “314‐type” Perovskite; a Highly Efficient Cathode for Solid Oxide Fuel Cells

“…On the other hand, there are two different crystallographic positions where the oxygen atom could be located in the oxygen deficient layers [(Co1/Fe1–O2, O3)] of the crystal structure, and this layer shows a significant oxygen deficiency. The refinements of the low temperature NPD patterns highlighted a Co 3+ high spin (HS, t 2g 4 e g 2 ) state and intermediate spin (IS, t 2g 5 e g 1 ) state ordering (antiferromagnetically aligned) in the oxygen replete layers . Therefore, the oxygen vacancy ordering in the oxygen‐deficient Co1/Fe1–O layers and the tilting of octahedra (Co1/Fe1‐O1‐Co2/Fe2 = 172.9°), due to the Jahn‐Teller distortion of the IS Co 3+ in the oxygen replete region, trigger the quadrupling of the c‐axis with respect to the oxygen disordered cubic cell.…”

“…However, above 700 °C the total oxygen content goes below 2.5. The linear thermal expansion coefficient (TEC) obtained from the high temperature XRD patterns for a similar “314‐type” compound with composition SrCo 0.75 Fe 0.25 O 2.64 (S1 in the supporting information, see also ) is α ∼ 19 × 10 −6 K −1 . Therefore we can expect the similar TEC value for the present material, and this is comparable to other cobalt‐containing perovskites .…”

“…Recently, we have demonstrated the stabilization of “314‐type” novel oxygen vacancy ordered phase without A‐site ordering . SrCo 0.85 Fe 0.15 O 2.62 is in fact the first example of “314‐type” structure in absence of A‐site ordering.…”

Room temperature magnetoresistance and exchange bias in “314-type” oxygen-vacancy ordered SrCo0.85Fe0.15O2.62

“…[11][12][13][14][15][16] A pure phase polycrystalline sample of SrCo 0.85 Fe 0.15 O 2.62 was prepared by standard solid-state reaction method as reported previously. 16 The sample was characterized by X-ray powder diffraction (XRD) at room temperature (RT), performed in a Panalytical a)…”