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

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.