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