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The thallium halides include monohalides, where thallium has oxidation state +1, trihalides in which thallium generally has oxidation state +3, and some intermediate halides containing thallium with mixed +1 and +3 oxidation states. These materials find use in specialized optical settings, such as focusing elements in research spectrophotometers. Compared to the more common zinc selenide-based optics, materials such as thallium bromoiodide enable transmission at longer wavelengths. In the infrared, this allows for measurements as low as 350 cm−1 (28 µm), whereas zinc selenide is opaque by 21.5 µm, and ZnSe optics are generally only usable to 650 cm−1 (15 µm).
The monohalides all contain thallium with oxidation state +1. Parallels can be drawn between the thallium(I) halides and their corresponding silver salts, for example thallium(I) chloride and bromide are light sensitive and thallium(I) fluoride is more soluble in water than the chloride and bromide.
TlF is a white crystalline solid, with a mp of 322 °C. It is readily soluble in water unlike the other Tl(I) halides. The normal room temperature form has a similar structure to α-PbO which has a distorted rock salt structure with essentially five coordinate thallium, the sixth fluoride ion is at 370 pm. At 62 °C it transforms to a tetragonal structure. This structure is unchanged up to pressure of 40 GPa.
The room temperature structure has been explained in terms of interaction between Tl 6s and the F 2p states producing strongly antibonding Tl-F states. The structure distorts to minimise these unfavourable covalent interactions.
At room temperature TlI is a yellow crystalline solid, mp 442 °C. The crystal structure is a distorted rock salt structure known as the β-TlI structure. At higher temperatures the colour changes to red with a structure the same as CsCl.
Thallium(I) mixed halides
Thallium bromoiodide and thallium bromochloride are mixed salts of thallium(I) that are used in spectroscopy as an optical material for transmission, refraction and focusing of infrared radiation. The materials were first grown by R. Koops in the laboratory of Olexander Smakula at the Carl Zeiss Optical Works, Jena in 1941. The red bromoiodide was coded KRS-5  and the colourless bromochloride, KRS-6  and this is how they are commonly known. The KRS prefix is an abbreviation of “Kristalle aus dem Schmelz-fluss”, (crystals from the melt). The compositions of KRS-5 and KRS-6 approximate to TlBr0.4I0.6 and TlBr0.3Cl0.7. KRS-5 is the most commonly used, its properties of being relatively insoluble in water and non-hygroscopic, make it an alternative to KBr, CsI and AgCl.
The thallium trihalides are less stable than their corresponding aluminium, gallium and indium counterparts and chemically quite distinct. The triiodide does not contain thallium with oxidation state +3 but is a thallium(I) compound and contains the linear triiodide (I3−) ion.
TlF3 is a white crystalline solid , mp 550 °C. The crystal structure is the same as YF3 and β-BiF3. In this the thallium atom is 9 coordinate,(tricapped trigonal prismatic). It can be synthesised by fluoridation of the oxide, Tl2O3, with F2, BrF3 or SF4 at 300 °C.
This unstable compound disproportionates at less than 40 °C to TlBr2. It can be prepared in CH3CN by treating a solution of TlBr with Br2 gas. In water the tetrahydrate complex can be prepared by adding bromine to a stirred suspension of TlBr.
TlI3 is a black crystalline solid prepared from TlI and I2 in aqueous HI. It does not contain thallium(III), but has the same structure as CsI3 containing the linear I3− ion.
Mixed valence halides
As a group these are not well characterised. They contain both Tl(I) and Tl(III), where the thallium(III) atom is present as complex anions e.g. TlCl4−.
This is formulated as TlITlIIICl4.
This yellow compound is formulated TlI3 TlIIICl6.
This compound is similar to Tl2Cl3 and is formulated TlI3TlIIIBr6
This pale brown solid is formulated TlITlIIIBr4
This compound has been reported as an intermediate in the synthesis of TlI3 from TlI and I2. The structure is not known.
Thallium(I) can form complexes of the type (TlX3)2− and (TlX4)3− both in solution and when thallium(I) halides are incorporated into alkali metal halides. These doped alkali metal halides have new absorption and emission nbands and are used as phosphors in scintillation radiation detectors.
Thallium(III) fluoride complexes
The salts NaTlF4 and Na3TlF6 do not contain discrete tetrahedral and octahedral anions. The structure of NaTlF4 is the same as fluorite (CaF2) with NaI and TlIII atoms occupying the 8 coordinate CaII sites. Na3TlF6 has the same structure as cryolite, Na3AlF6. In this the thallium atoms are octahedrally coordinated. Both compounds are usually considered to be mixed salts of Na+ and Tl3+.
Salts containing TlCl52− with a square pyramidal structure are known. Some salts that nominally contain TlCl52− actually contain the dimeric anion Tl2Cl104−, long chain anions where TlIII is 6 coordinate and the octahedral units are linked by bridging chlorine atoms, or mixed salts of TlIIICl4 and TlIIICl6.
The ion Tl2Cl93− where thallium atoms are octahedrally coordinated with three bridging chlorine atoms has been identified in the Caesium salt, Cs3Tl2Cl9.
Thallium(III) bromide complexes
Salts of TlIIIBr4− and TlIIIBr63− are known with various cations.
The TlBr52− anion has been characterised in a number of salts and is trigonal bipyramidal. Some other salts that nominally contain TlBr52− are mixed salts containing TlBr4− and Br−.
Thallium(III) iodide complexes
Salts of TlIIII4− are known. The TlIII anion is stable even though the triiodide is a thallium(I) compound.