Thallium halides

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Template:Short description The thallium halides include monohalides Template:Chem2, where thallium has oxidation state +1, trihalides Template:Chem2, where thallium generally has oxidation state +3, and some intermediate halides containing thallium with mixed +1 and +3 oxidation states. X is a halogen. These salts 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 zinc sulfide optics are generally only usable to 650 cm−1 (15 μm).

Monohalides

File:Thallium-iodide-3D-vdW.png
Thallium(I) iodide has the CsCl crystal structure.

The monohalides, also known as thallous halides, 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 melting point 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.[1]
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.[2]
Thallium(I) chloride
TlCl is a light sensitive, white crystalline solid, melting point 430 °C. The crystal structure is the same as CsCl.
Thallium(I) bromide
TlBr is a light sensitive, pale yellow crystalline solid, melting point 460 °C. The crystal structure is the same as CsCl.
Thallium(I) iodide
At room temperature, TlI is a yellow crystalline solid, melting point 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.[3]

Thallium(I) mixed halides

File:6 KRS5 Tallium Bromide Iodide ingots Crystaltechno.jpg
Thallium bromide iodide ingots

Thallium bromoiodide or thallium bromide iodide (Template:Chem2) and thallium bromochloride or thallium bromide chloride (Template:Chem2) 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.[4][5] The red bromoiodide was coded KRS-5[6] and the colourless bromochloride, KRS-6[7] 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 Template:Chem2 and Template:Chem2. 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.[8]

Trihalides

The thallium trihalides, also known as thallic halides, 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 Template:Chem2 ion.

Thallium(III) fluoride

TlF3 is a white solid, mp 550 °C. Its structure is the same as Template:Chem2 and β-Template:Chem2: 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

Template:Chem2 has a distorted chromium(III) chloride structure like Template:Chem2 and Template:Chem2. It can be prepared] by treating Template:Chem2 with Template:Chem2 gas.[9] Crystallization from water gives the tetrahydrate. Solid Template:Chem2 decomposes at 40 °C, losing chlorine to give Template:Chem2.

Thallium(III) bromide

Template:Chem2 can be prepared] by treating Template:Chem2 with Template:Chem2 gas.[10] Crystallization from water gives the tetrahydrate. Solid Template:Chem2 decomposes at 40 °C, losing bromine to give Template:Chem2.[11]

Thallium(I) triiodide

Template:Chem2 is a black crystalline solid prepared from Template:Chem2 and Template:Chem2 in aqueous HI. It does not contain thallium(III), but has the same structure as Template:Chem2 containing the linear Template:Chem2 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. Template:Chem2.

Template:Chem2
This is formulated as Template:Chem2.
Template:Chem2
This yellow compound is formulated Template:Chem2.[12]
Template:Chem2
This compound is similar to Template:Chem2 and is formulated Template:Chem2[13]
Template:Chem2
This pale brown solid is formulated Template:Chem2
Template:Chem2
This compound has been reported as an intermediate in the synthesis of Template:Chem2 from Template:Chem2 and Template:Chem2. The structure is not known.

Halide complexes

Thallium(I) complexes
Thallium(I) can form complexes of the type Template:Chem2 and Template:Chem2 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 Template:Chem2 and Template:Chem2 do not contain discrete tetrahedral and octahedral anions. The structure of Template:Chem2 is the same as fluorite (CaF2) with Template:Chem2 and Template:Chem2 atoms occupying the 8 coordinate Template:Chem2 sites. Na3TlF6 has the same structure as cryolite, Template:Chem2. In this the thallium atoms are octahedrally coordinated. Both compounds are usually considered to be mixed salts of Template:Chem2 and Template:Chem2.
Thallium(III) chloride complexes
Salts of tetrahedral Template:Chem2 and octahedral Template:Chem2 are known with various cations.
Salts containing Template:Chem2 with a square pyramidal structure are known. Some salts that nominally contain Template:Chem2 actually contain the dimeric anion Template:Chem2, long chain anions where Template:Chem2 is 6 coordinate and the octahedral units are linked by bridging chlorine atoms, or mixed salts of Template:Chem2 and Template:Chem2.[14]
The ion Template:Chem2, where thallium atoms are octahedrally coordinated with three bridging chlorine atoms, has been identified in the caesium salt, Template:Chem2.
Thallium(III) bromide complexes
Salts of Template:Chem2 and Template:Chem2 are known with various cations.
The Template:Chem2 anion has been characterised in a number of salts and is trigonal bipyramidal. Some other salts that nominally contain Template:Chem2 are mixed salts containing Template:Chem2 and Template:Chem2.[15]
Thallium(III) iodide complexes
Salts of Template:Chem2 are known. The Template:Chem2 anion is stable even though the triiodide is a thallium(I) compound.

References

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  6. ↑ Crystran Data for KRS5 https://www.crystran.co.uk/optical-materials/krs5-thallium-bromo-iodide-tlbr-tli
  7. ↑ Crystran Data for KRS6 https://www.crystran.co.uk/optical-materials/krs6-thallium-bromo-chloride-tlbr-tlcl
  8. ↑ Frank Twyman (1988) Prism and Lens Making: A Textbook for Optical Glassworkers CRC Press Template:ISBN, page 170
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Further information

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Template:Thallium compounds Template:Fluorine compounds

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