(H3O)3Sb2Br9: the first member of the M3E2X9 structure family with oxonium cations

(H3O)3Sb2Br9 [trihydroxonium enneabromidodiantimonate(III)] is the first representative of the M3E2X9 family (M = cation, E = Sb and Bi, and X = Br and I) with oxonium cations. The metastable compound was obtained in trace amounts from a solution of CsBr and SbBr3 in con­cen­trated aqueous HBr. Single crystals were isolated from the mother liquor and investigated by single-crystal X-ray diffraction at 100 K. (H3O)3Sb2Br9 crystallizes with the Tl3Bi2I9 structure type, which is a distorted defect variant of cubic perovskite. The crystal structure comprises characteristic 2∞[SbBr3Br3/2] double layers of corner-sharing SbBr6 octa­hedra with a [001] stacking direction. Due to the small size of the H3O+ cation and O—H⋯Br hydrogen bonding, the octa­hedra are tilted.