CRYSTAL-STRUCTURE, THERMAL AND MAGNETIC-PROPERTIES OF LA3CO3O8 - PHASE-RELATIONS FOR LACOO3-DELTA THAN-OR-EQUAL-TO-DELTA-LESS-THAN-OR-EQUAL-TO-0.50) AT 673 K

Phase relations including reoxidation and decomposition reactions were investigated for samples with nominal composition LaCoO3-delta (0.00 less than or equal to delta less than or equal to 0.50). LaCoO3-delta was synthesized by isothermal reduction at 673 K. For delta = 0.50 and 0.33 distinct phases occur, namely La2Co2O5 and La3Co3O8. La2Co2O5 ox idizes 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 react ions to CoO and the Ruddlesden-Popper type phases Lam+1ComO3m+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 P2(1), a = 555.93 pm, b = 541.51 pm, c = 1177.3 pm, be ta = 90.144 degrees 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 int ermediate between the brownmillerite and the perovskite type structure s. Double layers of corner-sharing CoO6 octahedra are connected by lay ers 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 n eutron diffraction data. The magnetic moments are mu(Co(1), tet) = 3.5 +/- 0.1 mu(B), mu(Co(2), oct) = 1.7 +/- 0.2 mu(B) and mu(Co(3), oct) = 2.0 +/- 0.2 mu(B).