Thorium triiodide is a binary inorganic compound of thorium metal and iodine with the chemical formula .
Th metal is heated with in a vacuum at 800 ðC.
Thorium triiodide is a black, violet-tinged, usually poorly crystallized mass. Formed crystals exhibit strong dichroism from violet to olive green under the microscope and are birefringent. Above 550 ðC, thorium(III) iodide decomposes to thorium(IV) iodide and thorium(II) iodide. ò-thorium(III) iodide has an orthorhombic crystal structure with the space group Cccm (space group no. 66). The ñ-form has a pseudoorthorhombic crystal structure.
reacts with water to form thorium tetraiodide and thorium metal.