Cation, Vacancy, and Spin Ordered 15R-Superstructures in Sr(Cr1‑xFe x )O3‑y (0.4 ≤ x ≤ 0.6) Perovskites

15-layer rhombohedral (15R) SrCrO2.8-type superstructures have been discovered in Sr(Cr1‑x Fe x )O3‑y perovskites (0.4 ≤ x ≤ 0.6; space group R3̅m; a ≈ 5.56 and c ≈ 34.6 Å). Cr/Fe cations are segregated between layers of tetrahedrally and octahedrally coordinated sites. The 15R-Sr(Cr1‑x Fe x )O3‑y materials are semiconducting and order ferrimagnetically below 225–342 K. The magnetic structure of an x = 0.5 sample shows spin canting consistent with a simple spin disorder model. Samples with x ≥ 0.7 have a disordered cubic perovskite structure, and we propose that locally reconstructed (111) planes like those in the 15R materials facilitate oxide ion migration in Cr-based perovskite mixed conductors used in solid oxide fuel cells.