(H 3 O) 3 Sb 2 Br 9 : the first member of the M 3 E 2 X 9 structure family with oxonium cations

(H 3 O) 3 Sb 2 Br 9 [trihydroxonium enneabromidodiantimonate(III)] is the first representative of the M 3 E 2 X 9 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 SbBr 3 in concentrated aqueous HBr. Single crystals were isolated from the mother liquor and investigated by single‐crystal X‐ray diffraction at 100 K. (H 3 O) 3 Sb 2 Br 9 crystallizes with the Tl 3 Bi 2 I 9 structure type, which is a distorted defect variant of cubic perovskite. The crystal structure comprises characteristic 2 ∞ [SbBr 3 Br 3/2 ] double layers of corner‐sharing SbBr 6 octahedra with a [001] stacking direction. Due to the small size of the H 3 O + cation and O—H…Br hydrogen bonding, the octahedra are tilted.