Thallium(I) iodide

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Thallium(I) iodide
Thallium(I) iodide
Other names
Thallium monoiodide
Thallous iodide
3D model (JSmol)
ECHA InfoCard 100.029.272
Molar mass 331.287 g/mol[1]
Appearance yellow crystals[1]
Density 7.1 g/cm3[1]
Melting point 441.7 °C (827.1 °F; 714.8 K)[1]
Boiling point 824 °C (1,515 °F; 1,097 K)[1]
0.085 g/L (25 °C)[1]
Solubility insoluble in alcohol[1]
−82.2·10−6 cm3/mol[2]
Very toxic (T+)
Dangerous for the environment (N)
R-phrases (outdated) R26/28, R33, R51/53
S-phrases (outdated) (S1/2), S13, S28, S45, S61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 is a chemical compound with the 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 [3] 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.[4] 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.[5] Thus, presence of impurities might be responsible for coexistence of the cubic and orthorombic TlI phases at ambient conditions.[3] Under high pressure, 160 kbar, TlI becomes a metallic conductor. Nanometer-thin TlI films grown on LiF, NaCl or KBr substrates exhibit the cubic rocksalt structure.[6]


Thallium(I) iodide is added to mercury arc lamps to improve their performance[7] 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.[8] Thallium(I) iodide is also used in trace amounts with NaI or CsI to produce scintillators used in radiation detectors.


Like all thallium compounds thallium(I) iodide is highly toxic.


  1. ^ a b c d e f g Haynes, p. 4.94
  2. ^ Haynes, p. 4.136
  3. ^ a b Lowndes, R. P.; Perry, C. H. (1973). "Molecular structure and anharmonicity in thallium iodide". The Journal of Chemical Physics. 58: 271. doi:10.1063/1.1678917. 
  4. ^ Mudring, Anja-Verena (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. 
  5. ^ Sultana, Saima; Rafiuddin (2009). "Electrical conductivity in TlI–TiO2 composite solid electrolyte". Physica B: Condensed Matter. 404: 36. doi:10.1016/j.physb.2008.10.002. 
  6. ^ Schulz, L. G. (1951). "Polymorphism of cesium and thallium halides". Acta Crystallographica. 4 (6): 487. doi:10.1107/S0365110X51001641. 
  7. ^ Reiling, Gilbert H. (1964). "Characteristics of Mercury Vapor–Metallic Iodide Arc Lamps". Journal of the Optical Society of America. 54 (4): 532. doi:10.1364/JOSA.54.000532. 
  8. ^ Underwater Journal and information bulletin, IPC Science and Technology Press, (1973), p 245

Cited sources[edit]