Our investigation of the Ba-Co-X (X = F, Cl, Br) systems has led to a number of new mixed-valent Co(III)/Co(IV) materials that have turned out to display complex magnetic properties. Here, we present a review of recent results about their crystallographic, magnetic and electric characteristics by comparison with several related compounds (including the BaCoO(3-delta) polytypes).From the structural point of view, the concerned compounds and their dimensionality can be deduced from each other by the reorganization of structural blocks isolated by anionic layers. These blocks contain linear (trimeric or tetrameric) cobalt based sub-units of primary importance in the field of pseudo-ID materials. We have investigated the particular dependence of the magnetic orderings on the connectivity of the concerned blocks and basic rules can be announced, highlighting the role of the inter-block connectivity on the local Co moments and on the sign and strength of the magnetic exchanges. (C) 2008 Elsevier Masson SAS. All rights reserved.

Ba6Co6ClO15.5 has been analyzed frorn the point of view of local electric/magnetic interactions and orbital overlapping, step by step following the different subunits of the crystal structure. Our investigation is based on structural data (X-ray and neutron diffraction, hereafter XRD and ND), experimental magnetic/electric/transport measurements, magnetic structure, and density functional theory (DFT) calculations. Its crystal structure is related to the 12H-BaCoO3-delta form and contains corner sharing pairs of Co4+ tetrahedra, with strong anti ferromagnetic (AF) exchanges (estimation of J similar to 0.07 eV). In the title compound, Co4+ are assigned to "intermediate spin" IS with electronic configuration e(g)(3)t(2g)(2), s = (3)/2. This rather rare configuration for a d(5) cation is explained from the splitting of the t(2g) manifold because of an important off-centering of the shared corner. The dimers are connected to linear Co43+O15 subunits, and both tetrahedral-tetrahedral and tetrahedral-octahedral magnetic junctions have been explored by the analysis of the correlation and delocalization Co-O-Co superexchanges, in good agreement with the experimental results. The tetrameric units have been assigned to an ordered mixture of HS and LS Co3+ while below T-N, and the localized magnetic moments vanished because of electron transfer toward covalent oxygen ligands and by direct exchanges in the intermediate Co-Co region. Then, the electrons are confined inside these units, which act as strong ferromagnetic connectors between the terminal Co4+ tetrahedra. The overview of the transport properties (conductivity, thermoelectric power, and positive magnetoresistance) are in favor of a variable range hoping (VRH) regime below T-N produced by an Anderson-like localization, in which tetrahedral Co4+ play the role of disordered centers with respect to a 1D system.

Parmi la grande diversite de composes formant la famille des perovskites hexagonales, les systemes a base de cobalt sont largement etudies a cause de leurs proprietes electroniques et magnetiques complexes. Ainsi, l'investigation des systemes Ba-Co-O-X avec X=F, Cl ou Br a permis de synthetiser de nouvelles phases dont les structures sont caracterisees par des groupements trimeres Co3O12 (trois octaedres CoO6 relies par une face) ou tetrameres Co4O15 (quatre octaedres CoO6 relies par une face). De fortes relations structurales ont ete mises en evidence entre les formes trimeres et tetrameres des composes oxydes, oxyfluorures, oxychlorures et oxybromures, notamment l'existence de desordres des atomes d'oxygene ou des transformations de phases a haute temperature (tetrameres -> trimeres). Dans tous ces materiaux, la couche d'interface entre blocs elementaires joue un role preponderant sur la dimensionnalite des structures creees. Par ailleurs, des mesures de susceptibilite magnetique combinees a des experiences de diffraction des neutrons ont permis d'etablir l'existence, en l'absence de champ magnetique, d'une mise en ordre antiferromagnetique selon l'axe c a basse temperature dans les materiaux halogeno-cobaltites. L'evolution des courbes d'aimantation en fonction du champ applique montre un comportement magnetique plus complexe pour les materiaux bromes, avec notamment la possibilite d'aligner les moments magnetiques dans le plan (ab) par application d'un champ magnetique. Finalement, ce travail de these presente les relations entre structure cristalline, dimensionnalite et proprietes magnetiques de ces nouvelles phases halogeno-cobaltites.

Single crystals of the title compounds were prepared by solid-solid reaction using BaBr(2) flux at 1373 K. The structures of these two new cobaltites were solved and refined. The two compounds are built from a close-packing of [BaO(3)] and [BaOBr] layers with stacking sequences (c'chhcc')(3) and (c'chhhcc')(2) for the 18R and 14H structures, respectively, which create Co(3)O(12) trimers or Co(4)O(15) tetramers of face-sharing octahedra connected at their extremities to isolated tetrahedra by corner-sharing. These new materials are strongly related to the 5H-Ba(5)Co(5)O(14)/12H-Ba(0.9)CoO(2.6) and 10H-Ba(5)Co(5)ClO(13)/6H-Ba(6)Co(6)ClO(16) materials, with the existence of common blocks. In Ba(6)Co(5)BrO(14) and Ba(7)Co(6)BrO(17), all the atoms in the vicinity of the [BaOBr] layers are disordered, whereas the rest of the structure is perfectly ordered.

Co 0.5 Ni 0.5 ) 3 V 2 O 8 represents a mixed compound of the well investigated transition metal (M) orthooxovanadates Ni 3 V 2 O 8 (NVO) and Co 3 V 2 O 8 (CVO) labelled as Kagomé staircase structures, which are characterized by edgesharing MO 6 octahedra isolated by nonmagnetic VO 4 tetrahedra.The crystallographic structure (orthorhombic space group Cmca) [1, 2] is interesting with respect to the magnetic properties as the magnetic ions form buckled planes of corner-sharing isosceles triangles representing an anisotropic variation of the ideal Kagomé net.Within these buckled planes, the Kagomé staircases, cross-tie ions on crystallographic (4a) sites link the linear chains of spine ions on (8e) sites.Due to the reduced symmetry of the Kagomé staircase geometry with respect to the ideal plane net the degree of frustration is lowered leading to interesting long range ordered magnetic structures.Magnetization and neutron diffraction experiments on a (Co 0.52 Ni 0.48 ) 3 V 2 O 8 powder sample [3] revealed only one magnetic phase transition into an antiferromagnetic ground state in contrast to the richness of magnetic phase transitions of its parent compounds [4,5].The magnetic structure is modulated by a composition dependent propagation vector k=(δ, 0, 0) with δ being 0.491(4) for (Co 0.52 Ni 0.48 ) 3 V 2 O 8 where a similarity to the NVO type magnetic structure was assumed [3].Neutron single crystal diffraction experiments followed by group theory analysis produced a more detailed picture.The magnetic structure of (Co 0.5 Ni 0.5 ) 3 V 2 O 8 exhibits features, which differ from the predominantly collinear alignment of its parent compounds NVO and CVO, which exhibit a variety of magnetic structures with magnetic moments mainly oriented along the a axis [4-7].The averaged magnetic moments of the statistically distributed Ni 2+ and Co 2 + ions are oriented in the a-c plane.They point either towards or away from the centers of the respective isosceles triangles of the Kagomé staircase structure if viewed as a projection along the b axis.The spin arrangement is close to a 120° configuration as expected for antiferromagnetically ordering systems on a Kagomé lattice.This result shows once again that the competition of the exchange interactions along various coupling pathways in this particular crystallographic system results in a variety of different interesting magnetic structures.

Single crystals of the title compounds were prepared by solid state reactions from barium carbonate and ruthenium metal using a BaBr2 flux and investigated by X-ray diffraction method using Mo(Kα) radiation and a Charge Coupled Device (CCD) detector. A structural model for the term n=2, Ba5Ru2Br2O9 (1) was established in the hexagonal symmetry, space group P63/mmc, a=5.8344(2)Å, c=25.637(2)Å, Z=2. Combined refinement and maximum-entropy method (MEM) unambiguously show the presence of CO32− ions in the three other compounds (2, 3, 4). Their crystal structures were solved and refined in the trigonal symmetry, space group P3¯ml, a=5.8381(1)Å, c=15.3083(6)Å for the term n=3, Ba6Ru3Br1.54(CO3)0.23O12 (2), and space group R3¯m, a=5.7992(1)Å, c=52.866(2)Å and a=5.7900(1)Å, c=59.819(2)Å for the terms n=4, Ba7Ru4Br1.46(CO3)0.27O15 (3), and n=5, Ba8Ru5Br1.64(CO3)0.18O18 (4), respectively. The structures are formed by the periodic stacking along [001] of (n+1) hexagonal close-packed [BaO3] layers separated by a double layer of composition [Ba2Br2−2x(CO3)x]. The ruthenium atoms occupy the n octahedral interstices created in the hexagonal perovskite slabs and constitute isolated dimers Ru2O9 of face-shared octahedra (FSO) in 1 and isolated trimers Ru3O12 of FSO in 2. In 3 and 4, the Ru2O9 units are connected by corners either directly (3) or through a slab of isolated RuO6 octahedra (4) to form a bidimensional arrangement of RuO6 octahedra. These four oxybromocarbonates belong to the family of compounds formulated [Ba2Br2−2x(CO3)x][Ban+1RunO3n+3] where n represents the thickness of the octahedral string in hexagonal perovskite slabs. These compounds are compared to the oxychloride series.

Single crystals of the title compounds were prepare d by solid-solid reaction using BaCl2 or BaBr2 flux at 1100°C. The structures of these two new co baltites were solved and refined in the trigonal symmetry with space group R 3m: a=5.716(2) Å, c=45.01(3) Å for Ba2Co4ClO7 and a=5.7434(5) Å, c=46.151(9) Å for Ba 2Co4BrO7. The two compounds are isostructural and their structures can be considere d as the intergrowth along [001] of hexagonal blocks (Ba 2Co8O14) 2built from a close-packing of [O 4] and [BaO3] layers with octahedral and tetrahedral cobalt, separated by flu orite-type double layers (Ba 2Cl2) 2+ or (Ba2Br2) . The main difference between Ba 2Co4ClO7 and Ba2Co4BrO7 is due to the fluoritetype layers: (Ba 2Cl2) 2+ double layers are perfectly ordered while (Ba 2 r2) 2+ blocks are affected by a structural disorder through the bromi ne atoms.

Single crystals of the title compounds were prepared by solid–solid reaction using BaBr 2 flux at 1373 K. The structures of these two new cobaltites were solved and refined. The two compounds are built from a close-packing of [BaO 3 ] and [BaOBr] layers with stacking sequences ( c ′ chhcc ′) 3 and ( c ′ chhhcc ′) 2 for the 18 R and 14 H structures, respectively, which create Co 3 O 12 trimers or Co 4 O 15 tetramers of face-sharing octahedra connected at their extremities to isolated tetrahedra by corner-sharing. These new materials are strongly related to the 5 H -Ba 5 Co 5 O 14 /12 H -Ba 0.9 CoO 2.6 and 10 H -Ba 5 Co 5 ClO 13 /6 H -Ba 6 Co 6 ClO 16 materials, with the existence of common blocks. In Ba 6 Co 5 BrO 14 and Ba 7 Co 6 BrO 17 , all the atoms in the vicinity of the [BaOBr] layers are disordered, whereas the rest of the structure is perfectly ordered.

Single crystals of the title compounds were prepared by solid-solid reaction using BaCl2 or BaBr2 flux at 1100 degrees C. The structures of these two new cobaltites were solved and refined in the trigonal symmetry with space group Rni:a = 5.716(2) angstrom, c = 45.01(3) A for Ba(2)CO(4)lO(7) and a = 5.7434(5) A, c = 46.151(9) A for Ba2Co4BrO7. The two compounds are isostructural and their structures can be considered as the intergrowth along [001] of hexagonal blocks (Ba2Co8O14)2- built from a close-packing Of [O-4] and [BaO3] layers with octahedral and tetrahedral cobalt, separated by fluorite-type double layers (Ba2Cl2)(2+) or (Ba2Br2)(2+). The main difference between Ba2Co4ClO7 and Ba2CO4BrO7 is due to the fluorite-type layers: (Ba2Cl2)(2+) double layers are perfectly ordered while (Ba2Br2)(2+) blocks are affected by a structural disorder through the bromine atoms.

Les structures magnetiques de deux nouveaux oxychlorures de baryum cobalt a valence mixte: Ba6Co6ClO16 (P-6m2, a=5,676(1) A et c=14,457(2) A) et Ba5Co5ClO13 (P63/mmc, a=5,698(1) A et c=24,469(5) A) ont ete resolues par diffraction des neutrons. Les structures nucleaires de ces composes sont particulierement similaires avec la presence de blocs oligomeres Co4O15 ou Co3O12 connectes entre eux via des dimeres tetraedriques Co2O7. Pour chacun des composes, des mesures de susceptibilite magnetique en fonction de la temperature ont montre l'existence d'une transition paramagnetique antiferromagnetique a basse temperature. Cette transition trouve son origine dans la mise en ordre antiferromagnetique des unites tetraedriques Co2O7.

Neutron diffraction (ND) Rietveld refinements at 3 K and ab initio density functional theory (DFT) calculations were performed on Ba6Co6ClO16−x, a new mixed valence cobalt oxychloride. The experimental data coincide with the theoretical ones to indicate that an antiferromagnetic ordering occurs. The refined magnetic structure displays antiferromagnetic Co2O7 tetrahedral dimers with a moment collinear to the c-axis, of 2.8 μB/Co atom (ND), the DFT giving 2.3 μB/Co. These results suggest that tetrahedral Co4+, in the intermediate spin configuration, are interconnected by tetrameric Co3+ octahedral units which are predicted by DFT to arrange ferromagnetically through delocalized electrons, in good agreement with ND.