The crystal and magnetic structures of LaCa₂Fe₃O ₈ and NdCa₂Fe₃O₈

The crystal and magnetic structures of LaCa₂Fe₃O ₈ and NdCa₂Fe₃O₈ have been established using a combination of x-ray, neutron and electron diffraction. It was already considered likely that LaCa₂Fe₃O₈ and NdCa₂Fe₃O₈ were made up of stacked perovskite-like layers of FeO₆ octahedra, with every third layer being replaced by a layer of tetrahedrally coordinated Fe, rather like a variation on the Brownmillerite (Ca₂Fe₂O₅) structure type. We have gone further and determined a likely space group for this Grenier phase and determined the magnetic structure of the compounds at room temperature. The space group is found to be P 2₁ma (b axis as the long axis), and the crystal structure has been refined, subject to the stacking faulting along the long axis that is apparent in electron diffraction patterns. The magnetic structure of LaCa₂Fe₃O₈ is shown to consist of antiferromagnetically ordered Fe³⁺ ions on a collinear G-type antiferromagnetic structure, with the magnetic moments most likely (anti)parallel with the c axis, and of magnitude 3.4 0.2μ_B (3.6 0.2μ_B for NdCa₂Fe₃O₈). The result is reasonable given the magnetic structures of the end members of the La_1-xCa_xFeO₃ series, LaFeO₃ (x = 0) and Ca₂Fe₂O₅ (x = 1).