Thallium(I) iodide
Thallium(I) iodide is a chemical compound with the formula TlI. It is unusual in being one of the few water-insoluble metal iodides, along with AgI, CuI, SnI2, SnI4, PbI2 and HgI2. Chemistry TlI can be formed in aqueous solution by metathesis of any soluble thallium salt with iodide ion. It is also formed as a by-product in thallium-promoted iodination of phenols with thallium(I) acetate. Attempts to oxidise TlI to thallium(III) iodide fail, since oxidation produces thallium(I) triiodide, . Physical properties The room temperature form of TlI is yellow and has an orthorhombic structure which can be considered to be a distorted NaCl structure. The distorted structure is believed to be caused by favourable thallium-thallium interactions, the closest Tl-Tl distance is 383 pm. At 175 °C the yellow form transforms to a red CsCl form. This phase transition is accompanied by about two orders of magnitude jump in electrical conductivity. The CsI structure can be stabilized down to room temperature by doping with other halides such as RbI, CsI, KI, AgI, TlBr and TlCl. Thus, presence of impurities might be responsible for coexistence of the cubic and orthorhombic phases at ambient conditions. Under high pressure, 160 kbar, becomes a metallic conductor.