title: Holmium(III) iodide
text: # Holmium(III) iodide
  
  Holmium(III) iodide is an iodide of holmium, with the chemical formula of $HoI_3.$ 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 $I_2$ → 2 $HoI_3$
  
  Holmium(III) iodide can also be obtained via the direct reaction between holmium and mercury(II) iodide:
  
      2 Ho + 3 $HgI_2$ → 2 $HoI_3$ + 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.
  
  