Bismuth(III) iodide is the chemical compound composed of bismuth and iodine with the formula BiI3. It forms gray to black hexagonal crystals and is used as a reagent in analytical chemistry. Bismuth(III) iodide forms a unique crystal structure, with iodine atoms occupying a hexagonally closest-packed lattice with bismuths occupying either one-third or two-thirds of the octahedral holes (alternating by layer).
As bismuth(III) iodide is insoluble in water, a solution can be tested for the presence of Bi3+ ions by adding a salt such as potassium iodide. A black precipitate of bismuth(III) iodide indicates a positive test.
Bismuth(III) iodide forms interesting iodobismuth(III) anions when heated with halide donors, such as: