Effect of Co Substitution on the Crystal and Magnetic Structure of SrFeO2.75-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 SrFe1-xCoxO2.75-δ (x = 0.1-0.85) materials. The introduction of Co in the parent SrFeO2.75 (Sr4Fe4O11) structure revealed an interesting structural transformation. At room temperature (RT), an orthorhombic (space group Cmmm, 2√2ap × 2ap × √2ap type, ap = lattice parameter of the cubic perovskite) → tetragonal (space group P4/mmm, ap × ap × 2ap type) → tetragonal (space group I4/mmm, 2ap × 2ap × 4ap 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 (ABO3-δ 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.