Ferrimagnetism as a Consequence of Unusual Cation Ordering in the Perovskite SrLa₂FeCoSbO₉

A polycrystalline sample of SrLa₂FeCoSbO₉ has been prepared in a solid-state reaction and studied by a combination of electron microscopy, magnetometry, Mössbauer spectroscopy, X-ray diffraction, and neutron diffraction. The compound adopts a monoclinic (space group P2₁/ n; a = 5.6218(6), b = 5.6221(6), c = 7.9440(8) Å, β = 90.050(7)° at 300 K) perovskite-like crystal structure with two crystallographically distinct six-coordinate sites. One of these sites is occupied by ²/₃ Co²⁺, ¹/₃ Fe³⁺ and the other by ²/₃ Sb⁵⁺, ¹/₃ Fe³⁺. This pattern of cation ordering results in a transition to a ferrimagnetic phase at 215 K. The magnetic moments on nearest-neighbor, six-coordinate cations align in an antiparallel manner, and the presence of diamagnetic Sb⁵⁺ on only one of the two sites results in a nonzero remanent magnetization of ∼1 μ_B per formula unit at 5 K.