Lanthanum also forms a diiodide, LaI. It is an electride and is best formulated {La,2I,e}, with the electron delocalised in a conduction band. Several other lanthanides form similar compounds, including CeI, PrI and GdI. Lanthanum diiodide adopts the same tetragonal crystal structure as PrI.
Lanthanum(III) iodide reacts with lanthanum metal under an argon atmosphere in a tantalum capsule at 1225 K to form the mixed-valence compound LaI.
Reduction of LaI or LaI with metallic sodium in an argon atmosphere at 550 °C gives lanthanum monoiodide, LaI, which has a hexagonal crystal structure.