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 TlI. It is unusual in being one of the few water-insoluble metal

iodide An iodide ion is the ion 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 defic ...

s, along with

AgI Silver iodide is an inorganic compound with the formula Ag I. The compound is a bright yellow solid, but samples almost always contain impurities of metallic silver that give a gray coloration. The silver contamination arises because AgI is hig ...

, CuI, SnI₂, SnI₄, PbI₂ and HgI₂.

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 In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of one or more hydroxyl groups (— O H) bonded directly to an aromatic hydrocarbon group. The simplest is phenol, . Phenolic compounds are c ...

with thallium(I) acetate. Attempts to

oxidise Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

TlI to thallium(III) iodide fail, since oxidation 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 lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...

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 383 pm. At 175 °C the yellow form transforms to a red

CsCl CSCL can refer to: * Caesium chloride (CsCl), a chemical compound. * Computer Supported Collaborative Learning, a research topic on supporting collaborative learning with the assistance of computer artifacts. * China Shipping Container Lines, a c ...

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 orthorombic 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, p ...

structure.

Applications

Thallium(I) iodide is 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. 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 absorbed ...

s used in radiation detectors.

Natural occurrence

Natural thallium(I) iodide was first discovered in a naturally occurring setting in 2017 as a orthorhombic polymorph 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. Opti ...

sourced from

fumarole A fumarole (or fumerole) is a vent in the surface of the Earth or other 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 Avachinsky (also known as Avacha or Avacha Volcano or Avachinskaya Sopka) (russian: Авачинская сопка, Авача) is an active stratovolcano in Russia. It is situated on the Kamchatka Peninsula in the Russian Far East. Avachinsky l ...

, a volcano in Russia's

Kamchatka Peninsula The Kamchatka Peninsula (russian: полуостров Камчатка, Poluostrov Kamchatka, ) 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 we ...

that can reach temperatures of . The geologists that discovered it speculate that further research into this mineral is likely to add to the understanding of the geochemical evolution of the planet

Chemistry

Physical properties

Applications

Natural occurrence

Safety

References

Cited sources