[BaCoO3]n[BaCo8O11] Modular Intergrowths: Singularity of the n = 2 Term

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 ∼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.