Holmium(III) iodide

Holmium(III) iodide is an iodide of holmium, with the chemical formula of HoI3. It is used as a component of metal halide lamps.

Preparation
Holmium(III) iodide can be obtained by directly reacting holmium and iodine:


 * 2 Ho + 3 I2 → 2 HoI3

Holmium(III) iodide can also be obtained via the direct reaction between holmium and mercury(II) iodide:


 * 2 Ho + 3 HgI2 → 2 HoI3 + 3 Hg

The mercury produced in the reaction can be removed by distillation.

Holmium(III) iodide hydrate can be converted to the anhydrous form by dehydration with a large excess of ammonium iodide (since the compound is prone to hydrolysis).

Properties
Holmium(III) iodide is a highly hygroscopic substance that dissolves in water. It forms yellow hexagonal crystals with a crystal structure similar to bismuth(III) iodide. In air, it quickly absorbs moisture and forms hydrates. The corresponding oxide iodide is also readily formed at elevated temperature.