Crystal Structure of AgBi 2 I 7 Thin Films.

Synthesis of cubic-phase AgBi2 I7 iodobismuthate thin films and fabrication of air-stable Pb-free solar cells using the AgBi2 I7 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 ThZr2 H7 crystal structure with AgBi2 I7 stoichiometry. Through careful examination of the proposed structure and computational evaluation of the phase stability and bandgap, we find that the reported "AgBi2 I7 " films cannot be forming with the ThZr2 H7 -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 ThZr2 I7 model. Our results provide critical insights for assessing the photovoltaic properties of AgBi2 I7 iodobismuthate materials.