Thallium(I) iodide is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with the formula . It is exists as both a solid and high temperature red polymorph. Thallium(I) iodide is one of several water-insoluble metal

iodide An iodide ion is I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine deficiency ...

s, along with AgI, CuI, SnI₂, SnI₄, PbI₂, and HgI₂.

Synthesis and reactions

TlI can be formed in aqueous solution by metathesis of soluble thallium salt with iodide ion. Alternatively, it has been prepared from the elements: : An excess of iodine produces thallium(I) triiodide, .

Physical properties

The room temperature form of TlI is yellow and has an

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic Lattice (group), lattices result from stretching a cubic crystal system, cubic lattice along two of its orthogonal pairs by two different factors, res ...

structure which can be considered to be a distorted NaCl structure. The distorted structure is may be caused by favorable thallium-thallium interactions, the closest Tl-Tl distance is 383 pm. 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 with other halides such as RbI, CsI, KI, AgI, TlBr and TlCl. Thus, presence of impurities might be responsible for coexistence of the cubic and orthorhombic phases at ambient conditions. Under high pressure, 160 kbar, becomes a metallic conductor. Nanometer-thin films grown on LiF, NaCl or KBr substrates exhibit the cubic

rocksalt Halite ( ), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pi ...

structure.

Applications

Thallium(I) iodide was initially added to mercury arc lamps to improve their performance The light produced was mainly in the blue green part of the visible light spectrum least absorbed by water, so these have been used for underwater lighting. In modern times, it is added to quartz and ceramic metal halide lamps that uses

rare-earth The rare-earth elements (REE), also called the rare-earth metals or rare earths, and sometimes the lanthanides or lanthanoids (although scandium and yttrium, which do not belong to this series, are usually included as rare earths), are a set of ...

halides like

dysprosium Dysprosium is a chemical element; it has symbol Dy and atomic number 66. It is a rare-earth element in the lanthanide series with a metallic silver luster. Dysprosium is never found in nature as a free element, though, like other lanthanides, it ...

, to increase their efficiency and to get the light color more close to the blackbody locus. Thallium iodide alone can be used to produces green colored metal halide lamps. Thallium(I) iodide is also used in trace amounts with NaI or CsI to produce

scintillator A scintillator ( ) is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the ab ...

s used in radiation detectors.

Natural occurrence

Thallium(I) iodide is a rare mineral called nataliyamalikite. Small grains were found embedded in

mascagnite Mascagnite is a rare ammonium sulfate mineral ( N H4)2 S O4. It crystallizes in the orthorhombic system typically forming as stalactitic masses exhibiting good cleavage. It is soft (not higher than 2.5 on the Mohs scale) and water-soluble. Optic ...

sourced from

fumarole A fumarole (or fumerole) is a vent in the surface of the Earth or another rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...

s at Avachinsky, a volcano in Russia's

Kamchatka Peninsula The Kamchatka Peninsula (, ) is a peninsula in the Russian Far East, with an area of about . The Pacific Ocean and the Sea of Okhotsk make up the peninsula's eastern and western coastlines, respectively. Immediately offshore along the Pacific ...

that can reach temperatures of .

Safety

Like all thallium compounds, thallium(I) iodide is highly toxic with an

LD50 In toxicology, the median lethal dose, LD50 (abbreviation for "lethal dose, 50%"), LC50 (lethal concentration, 50%) or LCt50 is a toxic unit that measures the lethal dose of a given substance. The value of LD50 for a substance is the dose requ ...

of 24.1mg/kg in

rats Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' (pack rats), '' Bandicota'' (bandicoot ...

References

Cited sources

* {{Iodides Iodides Thallium(I) compounds Metal halides