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[摘要]：The structure property relationships for the n = 2 member (Ba(3)Co(10)O(17)) of the [BaCoO(3)](n)[BaCo(8)O(11)] modular family have been studied by comparison to the n = 1 term (Ba(2)Co(9)O(14)). It orders antiferromagnetically at T(N) = 62 K with spin ordering in the common [BaCo(8)O(11)] units similar to that for n = 1. In the sandwiching perovskite [BaCoO(3)](n) blocks, the magnetic interactions are modified from n = 1 to 2 because of the large moment on the extra (Co3) cobalt atoms, while no local moment was detected on the perovskite modules for n = 1. For n = 2, it highlights the determinant role of Co3 in the magnetic paths and magnetocrystalline anisotropy. The transport and magnetic measurements on single crystals show a strong anomaly at similar to 160-170 K Below this temperature, the conductivity regime changes from a semiconducting behavior to a variable-range hopping conduction type. Possible symmetry losses have been investigated at 100 K. A trigonal-to-monoclinic-distortion is proposed but remains highly hypothetical. For both n = 1 and 2, magnetic/crystallographic considerations together with density functional theory calculations allow one to assume an almost satisfied charge Co(2+)/Co(3+) ordered distribution between all of the cobalt positions (ideal Co(2.7+) mean valence). However, hole doping appears the most probable feature to satisfy the CO(2.8+) deduced from the oxygen stoichiometry assigned from neutron diffraction. |
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