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 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 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 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, 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, 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.^[3]

Thallium(I) mixed halides

Thallium bromoiodide / thallium bromide iodide (TlBr_xI_1−x) and thallium bromochloride / thallium bromide chloride (TlBr_xCl_1−x) 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 TlBr_0.4I_0.6 and TlBr_0.3Cl_0.7. 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.^[8]

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 I−3 ion.

Thallium(III) fluoride

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.

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 TlBr and Br₂. 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.^[9]

Thallium(I) triiodide

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 I−3 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. TlCl−4.

TlCl₂

This is formulated as Tl^ITl^IIICl₄.

Tl₂Cl₃

This yellow compound is formulated Tl^I₃Tl^IIICl₆.^[10]

Tl₂Br₃

This compound is similar to Tl₂Cl₃ and is formulated Tl^I₃Tl^IIIBr₆^[11]

TlBr₂

This pale brown solid is formulated Tl^ITl^IIIBr₄

Tl₃I₄

This compound has been reported as an intermediate in the synthesis of TlI₃ from TlI and I₂. 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 Na^I and Tl^III atoms occupying the 8 coordinate Ca^II 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 TlCl−4 and octahedral TlCl3−6 are known with various cations.

Salts containing TlCl2−5 with a square pyramidal structure are known. Some salts that nominally contain TlCl2−5 actually contain the dimeric anion Tl₂Cl4−10, long chain anions where Tl^III is 6 coordinate and the octahedral units are linked by bridging chlorine atoms, or mixed salts of Tl^IIICl₄ and Tl^IIICl₆.^[12]

The ion Tl₂Cl3−9, 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 Tl^IIIBr−4 and Tl^IIIBr3−6 are known with various cations.

The TlBr2−5 anion has been characterised in a number of salts and is trigonal bipyramidal. Some other salts that nominally contain TlBr2−5 are mixed salts containing TlBr−4 and Br⁻.^[13]

Thallium(III) iodide complexes

Salts of Tl^IIII−4 are known. The Tl^III anion is stable even though the triiodide is a thallium(I) compound.