Crystal structure, thermal and magnetic properties of La₃Co₃O₈. Phase relations for LaCoO_3–δ (0.00≤δ≤0.50) at 673 K

Abstract

Phase relations including reoxidation and decomposition reactions were investigated for samples with nominal composition LaCoO_3–δ(0.00≤delta;≤0.50). LaCoO_3–δwas synthesized by isothermal reduction at 673 K. Forδ=0.50 and 0.33 distinct phases occur, namely La₂Co₂O₅ and La₃Co₃O₈ . La₂Co₂O₅ oxidizes via La₃Co₃O₈ to LaCoO₃ . None of these reduced, vacancy ordered phases show indications for any larger non-stoichiometry. La₂Co₂O₅ and La₃Co₃O₈ are metastable, and undergo irreversible decomposition reactions to CoO and the Ruddlesden–Popper type phases La_m+1Co_mO_3m+1,m=1 and 3, upon heating under inert atmosphere. The crystal structure of La₃Co₃O₈ is determined on the basis of simultaneous refinement of high-resolution powder X-ray diffraction and neutron diffraction data. The space group is P2₁,a=555.93 pm, b=541.51 pm, c=1177.3 pm, β=90.144° at RT; R_wp(PXD)=4.6%,R_wp(PND)=6.1%, R_p(PXD)=3.6%,R_p(PND)=4.7%. La₃Co₃O₈ adopts a structure which is an intermediate between the brownmillerite and the perovskite type structures. Double layers of corner-sharing CoO₆ octahedra are connected by layers with chains of corner-sharing CoO₄ tetrahedra running along [010]. La₃Co₃O₈ orders antiferromagnetically at temperatures below T_N=35 K. A probable magnetic structure is described on the basis of powder neutron diffraction data. The magnetic moments are µ_{Co(1), tet}=3.5±0.1 µ_B , µ_{Co(2), oct}=1.7±0.2 µ_B and µ_{Co(3), oct}=2.0±0.2 µ_B .

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