Thorium triiodideThorium triiodide is a binary inorganic compound of thorium metal and iodine with the chemical formula.SynthesisTh metal is heated with [Thorium(IV) iodide|] in a vacuum at 800 °C.Physical propertiesThorium 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 °C, thorium iodide decomposes to thorium iodide and thorium iodide. β-thorium iodide has an orthorhombic [crystal structure] with the space group Cccm. The α-form has a pseudoorthorhombic crystal structure.Chemical propertiesreacts with water to form thorium tetraiodide and thorium metal.