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.

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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⁻.

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.^[1] At 175^oC 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 ^[2] 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.^[3]
• 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.

See also

• Thallium halides
• Thallium(I) chloride

General references

• WebElements
• Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.