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Thallium(I) iodide

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Formula
  
TlI

Molar mass
  
331.29 g/mol

Thallium(I) iodide httpsuploadwikimediaorgwikipediacommons77

Related compounds
  
Mercury(II) iodide Lead(II) iodide

Appearance
  
yellow crystals; red crystals above 170 °C

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, SnI2, SnI4, PbI2 and HgI2.

Contents

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 the thallium(I) triiodide, Tl+I3.

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 383pm. 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 TlI with other iodides such as RbI, CsI, KI, AgI, TlBr and TlCl. Thus, presence of impurities might be responsible for coexistence of the cubic and orthorombic TlI phases at ambient conditions. Under high pressure, 160kbar, TlI becomes a metallic conductor.

Applications

  • 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.
  • Safety

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

    References

    Thallium(I) iodide Wikipedia