Thallium(I) iodide

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Thallium(I) iodide
Thallium(I) iodide
Other names
Thallium monoiodide
Thallous iodide
7790-30-9 YesY
ChemSpider 56430 YesY
Jmol 3D model Interactive image
PubChem 62679
Molar mass 331.29 g/mol
Appearance yellow crystals
red crystals above 170 °C
Density 7.29 g/cm3, solid
Melting point 442 °C (828 °F; 715 K)
Boiling point 823 °C (1,513 °F; 1,096 K)
Very toxic (T+)
Dangerous for the environment (N)
R-phrases R26/28, R33, R51/53
S-phrases (S1/2), S13, S28, S45, S61
Flash point Non-flammable
Related compounds
Other anions
Thallium(I) fluoride
Thallium(I) chloride
Thallium(I) bromide
Other cations
Gallium(I) iodide
Indium(I) iodide
Related compounds
Mercury(II) iodide
Lead(II) iodide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
Infobox references

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.


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[edit]

The room temperature form of TlI is yellow and has an orthorhombic structure [1] 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.[2] 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.[3] Thus, presence of impurities might be responsible for coexistence of the cubic and orthorombic TlI phases at ambient conditions.[1] Under high pressure, 160kbar, TlI becomes a metallic conductor.


  • Added to mercury arc lamps to improve their performance[4] 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.[5]
  • 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[edit]

General references[edit]


  1. ^ a b R. P. Lowndes and C. H. Perry "Molecular structure and anharmonicity in thallium iodide" J. Chem. Phys. 58 (1973) 271
  2. ^ Anja-Verena Mudring (2007). "Thallium Halides - New Aspects of the Stereochemical Activity of Electron Lone Pairs of Heavier Main-Group Elements". European Journal of Inorganic Chemistry 2007 (6): 882–890. doi:10.1002/ejic.200600975. 
  3. ^ S. Sultana et al. "Electrical conductivity in TlI–TiO2 composite solid electrolyte" Physica B: 404 (2009) 36
  4. ^ G. H. Reiling, "Characteristics of Mercury Vapor-Metallic Iodide Arc Lamps," J. Opt. Soc. Am. 54, 532 (1964)
  5. ^ Underwater Journal and information bulletin, IPC Science and Technology Press, (1973), p 245