KRS-5

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).

Monohalides

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.

; Thallium(I) fluoride
:TlF is a white crystalline solid, with a mp of 322 °C. It is readily soluble in water unlike the other halides. The normal room temperature form has a similar structure to 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.

; Thallium(I) chloride
:TlCl is a light sensitive, white crystalline solid, mp 430 °C. The crystal structure is the same as CsCl.
;Thallium(I) bromide
:TlBr is a light sensitive, pale yellow crystalline solid, mp 460 °C. The crystal structure is the same as CsCl.

; Thallium(I) iodide
:At room temperature is a yellow crystalline solid, mp 442 °C. The crystal structure is a distorted rock salt structure known as the 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 and . 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.

Trihalides

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.

;Thallium(III) fluoride
:TlF₃ is a white crystalline solid , mp 550 °C. The crystal structure is the same as YF₃ and . 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.

;Thallium(III) chloride
:TlCl₃ has a distorted Cr(III) chloride structure like AlCl₃ and InCl₃. Solid TlCl₃ is unstable and disproportionates at 40 °C, losing chlorine to give TlCl. It can be prepared in CH₃CN by treating a solution of TlCl with Cl₂ gas.

;Thallium(III) bromide
: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.

; Thallium(I) triiodide
: is a black crystalline solid prepared from and 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 .

;
:This yellow compound is formulated .

;
:This compound is similar to and is formulated

;
:This pale brown solid is formulated

;
:This compound has been reported as an intermediate in the synthesis of from and . The structure is not known.

Halide complexes

;Thallium(I) complexes
: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 and atoms occupying the 8 coordinate 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⁺.

;Thallium(III) chloride complexes
:Salts of tetrahedral TlCl₄⁻ and octahedral TlCl₆³⁻are known with various cations.

: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 is 6 coordinate and the octahedral units are linked by bridging chlorine atoms, or mixed salts of and .

: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 and 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 are known. The anion is stable even though the triiodide is a thallium(I) compound.

References

Further information

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{{fluorine compounds

Thallium compounds
Metal halides
Mixed valence compounds