Crystal Structures ofMBi2Br7(M= Rb, Cs) - Filled Variants ofAX7Sphere Packing

The crystal structures of α-Rb7Sb3Br16, α- and β-Tl7Bi3Br16 and their relationship to close packings of spheres

Yellow prismatic crystals of rubidium bromido-antimonate(III) Rb7Sb3Br16 and of two different modifications of thallium bromido-bismuthate(III) Tl7Bi3Br16 were obtained by solvent-free synthesis and by precipitation from acidic aqueous solutions. X-ray diffraction analyses revealed the Tl7Bi3I16-type for α-Tl7Bi3Br16 (orthorhombic, Cmcm, a = 2324.31(8) pm, b = 1346.69(4) pm, c = 3460.0(1) pm; Pearson symbol oC312) and a new structure type for β-Tl7Bi3Br16 (monoclinic, C2/c, a = 2331.87(5) pm, b = 1343.33(3) pm, c = 3546.01(7) pm, β = 102.708(1)°; mC312). The antimonate Rb7Sb3Br16 adopts the Tl7Bi3I16-type, too (orthorhombic, Cmcm, a = 2347.16(3) pm, b = 1357.89(5) pm, c = 3539.47(9) pm; oC312). The crystal structures of α- and β-Tl7Bi3Br16 comprise alternating slabs of isolated [BiBr6]3– octahedra and [Bi2Br10]4– octahedra pairs. Both structure types are hierarchically organized and can be regarded as sphere close packing with the same stacking sequence, if octahedra and octahedra pairs are replaced by spheres of equal size. The structural relationship between the Tl7Bi3I16-type and the hydrate Na7Bi3Br16 · 18H2O, which comprises similar structural features, is discussed.

Crystal Structure of AgBi2I7 Thin Films

Synthesis of cubic-phase AgBi₂I₇ iodobismuthate thin films and fabrication of air-stable Pb-free solar cells using the AgBi₂I₇ absorber have recently been reported. On the basis of X-ray diffraction (XRD) analysis and nominal composition, it was suggested that the synthesized films have a cubic ThZr₂H₇ crystal structure with AgBi₂I₇ stoichiometry. Through careful examination of the proposed structure and computational evaluation of the phase stability and bandgap, we find that the reported "AgBi₂I₇" films cannot be forming with the ThZr₂H₇-type structure, but rather more likely adopt an Ag-deficient AgBiI₄ type. Both the experimental X-ray diffraction pattern and bandgap can be better explained by the AgBiI₄ structure. Additionally, the proposed AgBiI₄ structure, with octahedral bismuth coordination, removes unphysically short Bi-I bonding within the [BiI₈] hexahedra of the ThZr₂I₇ model. Our results provide critical insights for assessing the photovoltaic properties of AgBi₂I₇ iodobismuthate materials.