Iodine Close Packing in Hybrid Halide Bismuth(III) and Antimony(III) Semiconductors: (NH3(CH2)7NH3)2Bi2I10 and (NH3(CH2)7NH3)2Sb2I10.

Simple features in complex hybrid inorganic-organic crystalline materials provide opportunities for targeted discovery of materials with desired optoelectronic properties. In this study, we report the structure and optoelectronic properties of isostructural (NH3(CH2)7NH3)2Bi2I10 and (NH3(CH2)7NH3)2Sb2I10. The crystal structures are characterized by corner-connected metal-iodide octahedral chains that form a cubic close-packed iodine inorganic framework. Variable temperature UV-visible diffuse reflectance spectroscopy reveals stark contrasts in the onset of absorption and color changes between the [MX6]3- based structures, due to differences in the interaction of the (NH3(CH2)7NH3)2+ organic ammonium cation and the iodine packing of the inorganic framework. Density functional theory (DFT) calculations reveal flat bands reflective of the pseudo-1D crystal structure. Dark microwave conductivity (DMC) and time-resolved microwave conductivity (TRMC) reveal an excitonic character with long carrier lifetimes, consistent with the electronically confined octahedral chains. Comparison of the structural features with those of other diammonium-containing crystals reveals that diammoniumheptane can substitute into structures, displacing inorganic octahedra while retaining a close-packed anion framework. This provides a means for targeting new hybrid materials in which "vacancy-ordering" provides a crystal chemical approach for targeting desirable optoelectronic properties.