Room-temperature crystal structures of [CH(NH₂)₂]₃Sb₂X₉ (X = Br and I)

Crystals of formamidinium antimony, halides, FA₃Sb₂X₉ {FA = [CH(NH₂)₂]⁺; X = Br^- and I^-} {or triformamidinium nonahalidodiantimony, (CH₅N₂)₃[Sb₂X₉]}, have been synthesized using a counter diffusion crystal growth (CDCG) method in silica gel and their structures determined from single-crystal X-ray diffraction data. FA₃Sb₂Br₉ belongs to the trigonal space group P3m1, which is known as the Cs₃Bi₂Br₉ structure type, and FA₃Sb₂I₉ belongs to the hexagonal space group P6₃/mmc, called the Cs₃Cr₂Cl₉ structure type. The change of the anion type from bromide to iodide results in the change of the structure type and the connectivity of the Sb-X octahedra. These structures are described and compared to the crystal types known for vacancy-ordered triple-perovskites.