Layered ruthenium hexagonal perovskites: The new series [Ba2Br2-2x(CO3)x][Ban+1RunO3n+3] with n = 2, 3, 4, 5

International audienceSingle 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(Ka) radiation and a Charge Coupled Device (CCD) detector. A structural model for the term n ¼ 2, Ba5Ru2Br2O9 (1) was established in the hexagonal symmetry, space group P63/mmc, a ¼ 5.8344(2)A ˚ , c ¼ 25.637(2)A ˚ , Z ¼ 2. Combined refinement and maximum-entropy method (MEM) unambiguously show the presence of CO3 2 ions in the three other compounds (2, 3, 4). Their crystal structures were solved and refined in the trigonal symmetry, space group P3ml, a ¼ 5.8381(1)A ˚ , c ¼ 15.3083(6)A ˚ for the term n ¼ 3, Ba6Ru3Br1.54(CO3)0.23O12 (2), and space group R3m, a ¼ 5.7992(1)A ˚ , c ¼ 52.866(2)A ˚ and a ¼ 5.7900(1)A ˚ , c ¼ 59.819(2)A ˚ for the terms n ¼ 4, Ba7Ru4Br1.46(CO3)0.27O15 (3), and n ¼ 5, Ba8Ru5Br1.64(CO3)0.18O18 (4), respectively. The structures are formed by the periodic stacking along [0 0 1] of (n+1) hexagonal closepacked [BaO3] layers separated by a double layer of composition [Ba2Br22x(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 [Ba2Br22x(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

Mise en ordre magnétique dans de nouveaux oxychlorures de baryum-cobalt à valences mixtes

National audienceLes structures magnétiques de deux nouveaux oxychlorures de baryum cobalt à valence mixte: Ba6Co6ClO16 (P-6m2, a=5,676(1) Å et c=14,457(2) Å) et Ba5Co5ClO13 (P63/mmc, a=5,698(1) Å et c=24,469(5) Å) ont été résolues par diffraction des neutrons. Les structures nucléaires de ces composés sont particulièrement similaires avec la présence de blocs oligomères Co4O15 ou Co3O12 connectés entre eux via des dimères tétraédriques Co2O7. Pour chacun des composés, des mesures de susceptibilité magnétique en fonction de la température ont montré l'existence d'une transition paramagnétique antiferromagnétique à basse température. Cette transition trouve son origine dans la mise en ordre antiferromagnétique des unités tétraédriques Co2O7

Structure, dimensionnalité et magnétisme de nouvelles halogéno-cobaltites

Among the wide diversity of hexagonal perovskite-related materials, cobalt-based systems have been widely studied because of their complex electronic and magnetic properties. The investigation of systems Ba-Co-O-X with X=F, Cl or Br has led to the synthesis of new phases with structures characterized by Co3O12 trimers (three face-shared CoO6 octahedra) or Co4O15 tetramers (four face-shared CoO6 octahedra) entities. Strong structural relationships have been pointed out between trimers and tetramers forms for oxides, oxyfluorides, oxychlorides and oxybromides compounds, with for example oxygen disorders or high temperature phases transformations (tetramers -> trimers). In these materials, the interface layer between elementary blocks is of particular importance for the dimensionality of the structures. Magnetic susceptibility measurements combined to neutrons diffraction experiments have established a paramagnetic -> antiferromagnetic transition along the c-axis in the low temperature range for halogeno-cobaltites materials. Isothermal magnetic field dependences of the magnetization have shown a more complex magnetic behaviour for the bromed materials with the possibility to align the magnetic moments in the (ab) plane by application of a magnetic field. Finally, this work deals with the relations between crystal structure, dimensionality and magnetic properties of these new halogeno-cobaltites phases.Parmi la grande diversité de composés formant la famille des perovskites hexagonales, les systèmes à base de cobalt sont largement étudiés à cause de leurs propriétés électroniques et magnétiques complexes. Ainsi, l'investigation des systèmes Ba-Co-O-X avec X=F, Cl ou Br a permis de synthétiser de nouvelles phases dont les structures sont caractérisées par des groupements trimères Co3O12 (trois octaèdres CoO6 reliés par une face) ou tétramères Co4O15 (quatre octaèdres CoO6 reliés par une face). De fortes relations structurales ont été mises en évidence entre les formes trimères et tétramères des composés oxydes, oxyfluorures, oxychlorures et oxybromures, notamment l'existence de désordres des atomes d'oxygène ou des transformations de phases à haute température (tétramères -> trimères). Dans tous ces matériaux, la couche d'interface entre blocs élémentaires joue un rôle prépondérant sur la dimensionnalité des structures créées. Par ailleurs, des mesures de susceptibilité magnétique combinées à des expériences de diffraction des neutrons ont permis d'établir l'existence, en l'absence de champ magnétique, d'une mise en ordre antiferromagnétique selon l'axe c à basse température dans les matériaux halogéno-cobaltites. L'évolution des courbes d'aimantation en fonction du champ appliqué montre un comportement magnétique plus complexe pour les matériaux bromés, avec notamment la possibilité d'aligner les moments magnétiques dans le plan (ab) par application d'un champ magnétique. Finalement, ce travail de thèse présente les relations entre structure cristalline, dimensionnalité et propriétés magnétiques de ces nouvelles phases halogéno-cobaltites

New cobaltite materials containing CdI2-type layers: Synthesis and structures of Ba2Co4ClO7 and Ba2Co4BrO7

Single crystals of the title compounds were prepared by solid-solid reaction using BaCl2 or BaBr2 flux at 1100 C. The structures of these two new cobaltites were solved and refined in the trigonal symmetry with space group R3m: a ¼ 5.716(2) A° , c ¼ 45.01(3) A° for Ba2Co4ClO7 and a ¼ 5.7434(5) A° , c ¼ 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 [O4] and [BaO3] layers with octahedral and tetrahedral cobalt, separated by fluorite-type double layers (Ba2Cl2)2þ or (Ba2Br2)2þ. The main difference between Ba2Co4ClO7 and Ba2Co4BrO7 is due to the fluorite-type layers: (Ba2Cl2)2þ double layers are perfectly ordered while (Ba2Br2)2þ blocks are affected by a structural disorder through the bromine atoms

New oxybromide cobaltites with layered perovskite-related structures: 18 R-Ba6Co5BrO14 and 14H-Ba7Co6BrO17

International audienceSingle crystals of the title compounds were prepared by solid– solid reaction using BaBr2 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 [BaO3] and [BaOBr] layers with stacking sequences (c0chhcc0)3 and (c0chhhcc0)2 for the 18R and 14H structures, respectively, which create Co3O12 trimers or Co4O15 tetramers of facesharing octahedra connected at their extremities to isolated tetrahedra by corner-sharing. These new materials are strongly related to the 5H-Ba5Co5O14/12H-Ba0.9CoO2.6 and 10H-Ba5Co5ClO13/6H-Ba6Co6ClO16 materials, with the existence of common blocks. In Ba6Co5BrO14 and Ba7Co6BrO17, all the atoms in the vicinity of the [BaOBr] layers are disordered, whereas the rest of the structure is perfectly ordered

Magnetic structure of Ba7Co6BrO17

International audienceAt the basis of the search for original electronic and magnetic behaviour, a number of studies focus on the prospecting for new complex structural types within “hot” topics such as cobaltites (strong competition between magnetic ground states, attractive thermoelectricity, superconductivity ...). In this context, we recently synthesized by solid state reaction the new Ba7Co6BrO17 compound (P63/mmc: a=5.6611(1) Å and c=33.5672(8) Å) [1]. This cobaltite is built from a close-packing of [BaO3] and [BaOBr] layers with a 14H stacking sequence (c'chhhcc')2, which creates Co4O15 tetramers of face-sharing octahedra connected to their extremities to isolated tetrahedra by corner-sharing. This material is strongly related to the 12H-BaCoO2.6 [2] and 6H-Ba6Co6ClO16 [3] cobaltites, with the existence of common blocks. Measurement of the magnetic susceptibility Χ against T has been performed under applied magnetic field of 1 T. The experiment reveals the existence of two transitions at 60 K and 30 K. The curve was fitted to the Curie-Weiss law in the paramagnetic domain (above 60 K), leading to the values peff=3.12 ΜB/Co and Θ=55K This paramagnetic effective moment can be explained by assuming Co3+ S=1 and Co4+ S=3/2 i.e. cobalt atoms at the intermediate spin-state (considering the spin-only approximation). The positive Θ value indicates important ferromagnetic exchanges among the structures. In order to understand the magnetic properties of this cobaltite, we collected neutron powder diffraction data on the G41 diffractometer (LLB Saclay, France). The pattern below 60 K revealed the onset of new reflections, this fact being ascribed to the occurrence of long-range magnetic ordering and confirming the magnetic transition previously reported. All those magnetic reflections were indexed in a commensurate lattice related to the crystallographic one by a propagation vector k=[000]. Among all the possible magnetic models, only those for which the exchange interactions between two tetrahedral cobalt orders antiferromagnetically went to convergence with acceptable reliability factors. The magnetic structure can be described as the antiferromagnetic coupling of ferromagnetic blocks (Co4O15 + 2 CoO4 units). This kind of magnetic structure has been previously reported for 6H-Ba6Co6ClO16 [4]

Structure, dimensionnalité et magnétisme de nouvelles halogéno-cobaltites

Parmi la grande diversité de composés formant la famille des perovskites hexagonales, les systèmes à base de cobalt sont largement étudiés à cause de leurs propriétés électroniques et magnétiques complexes. Ainsi, l'investigation des systèmes Ba-Co-O-X avec X=F, Cl ou Br a permis de synthétiser de nouvelles phases dont les structures sont caractérisées par des groupements trimères Co3Ol2 (trois octaèdres CoO6 reliés par une face) ou tétramères Co4O15 (quatre octaèdres CoO6 reliés par une face). De fortes relations structurales ont été mises en évidence entre les formes trimères et tétramères des composés oxydes, oxyfluorures, oxychlorures et oxybromures, notamment l'existence de désordres des atomes d'oxygène ou des transformations de phases à haute température (tétramères -> trimères). Dans tous ces matériaux, la couche d'interface entre blocs élémentaires joue un rôle prépondérant sur la dimensionnalité des structures créées. Par ailleurs, des mesures de susceptibilité magnétique combinées à des expériences de diffraction des neutrons ont permis d'établir l'existence, en l'absence de champ magnétique, d'une mise en ordre antiferromagnétique selon l'axe c à basse température dans les matériaux halogéno-cobaltites. L'évolution des courbes d'aimantation en fonction du champ appliqué montre un comportement magnétique plus complexe pour les matériaux bromés, avec notamment la possibilité d'aligner les moments magnétiques dans le plan (ab) par application d'un champ magnétique. Finalement, ce travail de thèse présente les relations entre structure cristalline, dimensionnalité et propriétés magnétiques de ces nouvelles phases halogéno-cobaltites.LILLE1-BU (590092102) / SudocSudocFranceF

Magnetorelaxometry using Improved Giant MagnetoResistance Magnetometer

International audienceMagnetorelaxometry measurements on immobilized superparamagnetic magnetite (Fe3O4) nanoparticles in freeze-dried samples using a low-cost Improved Giant MagnetoResistance Magnetometer (IGMRM) are presented. Fits to relaxation phenomenological model based on Moment Superposition Model (MSM) are used to extract the characteristic values. Concentrations of iron down to 3.85 mol are identified in a 150 l volume from experimental measurements. Also, the extracted amplitude characteristic value confidence intervals associated to measurements and detection performances are given