Thallium(I) iodide

Thallium(I) iodide (TlI) is a chemical compound of formula TlI. It is unusual in being one of the few water-insoluble metal iodides, along with AgI, PbI2 and HgI2.

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 the thallium(I) triiodide, Tl+I3&minus;.

Yellow TlI 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 383pm. At 175oC it transforms to a red CsCl form. Under high pressure, 160kbar, it becomes a metallic conductor. Applications include:
 * Added to mercury arc lamps to improve their performance The light produced is mainly in the blue green part of the visible light spectrum least absorbed by water, so these have been used for underwater lighting.
 * Used in trace amounts with NaI or CsI to produce a scintillator used in radiation scintillation detectors.

Thallium(I) iodide is, like all thallium compounds, highly toxic. Conditions/substances to avoid are: heat.

General references

 * WebElements