# Thallium halides

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, 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.
    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, 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.

## Thallium(I) mixed halides

Thallium bromoiodide / thallium bromide iodide (TlBr$_x$I$_{1−x}$) and thallium bromochloride / thallium bromide chloride (TlBr$_x$Cl$_{1−x}$) 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. 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 TlBr$_{0.4}$I$_{0.6}$ and TlBr$_{0.3}$Cl$_{0.7}$. 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.

## 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 I−3 ion.

### Thallium(III) fluoride

    TlF$_3$ is a white solid, mp 550 °C. Its structure is the same as YF$_3$ and β-BiF$_3$: thallium atom is 9 coordinate (tricapped trigonal prismatic). It can be synthesised by fluoridation of the oxide, Tl$_2$O$_3$, with F$_2$, BrF$_3$, or SF$_4$ at 300 °C.

### Thallium(III) chloride

    TlCl$_3$ has a distorted Cr(III) chloride structure like AlCl$_3$ and InCl$_3$. It can be prepared] by treating TlCl with Cl$_2$ gas. Crystallization from water gives the tetrahydrate. Solid TlCl$_3$ decomposes at 40 °C, losing chlorine to give TlCl.

### Thallium(III) bromide

       TlBr$_3$ can be prepared] by treating TlBr with Br$_2$ gas. Crystallization from water gives the tetrahydrate. Solid TlBr$_3$ decomposes at 40 °C, losing bromine to give TlBr.

### Thallium(I) triiodide

    TlI$_3$ is a black crystalline solid prepared from TlI and I$_2$ in aqueous HI. It does not contain thallium(III), but has the same structure as CsI$_3$ 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.

### TlCl$_2$

    This is formulated as Tl$^I$Tl$^{III}$Cl$_4$.

### Tl$_2$Cl$_3$

    This yellow compound is formulated Tl$^I$$_3$Tl$^{III}$Cl$_6$.

### Tl$_2$Br$_3$

    This compound is similar to Tl$_2$Cl$_3$ and is formulated Tl$^I$$_3$Tl$^{III}$Br$_6$

   ### TlBr$_2$

    This pale brown solid is formulated Tl$^I$Tl$^{III}$Br$_4$

   ### Tl$_3$I$_4$

    This compound has been reported as an intermediate in the synthesis of TlI$_3$ from TlI and I$_2$. The structure is not known.

## Halide complexes
### Thallium(I) complexes

    Thallium(I) can form complexes of the type (TlX$_3$)$^{2−}$ and (TlX$_4$)$^{3−}$ 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 NaTlF$_4$ and Na$_3$TlF$_6$ do not contain discrete tetrahedral and octahedral anions. The structure of NaTlF$_4$ is the same as fluorite (CaF$_2$) with Na$^I$ and Tl$^{III}$ atoms occupying the 8 coordinate Ca$^{II}$ sites. Na$_3$TlF$_6$ has the same structure as cryolite, Na$_3$AlF$_6$. In this the thallium atoms are octahedrally coordinated. Both compounds are usually considered to be mixed salts of Na$^+$ and Tl$^{3+}$.

### 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 Tl$_2$Cl4−10, long chain anions where Tl$^{III}$ is 6 coordinate and the octahedral units are linked by bridging chlorine atoms, or mixed salts of Tl$^{III}$Cl$_4$ and Tl$^{III}$Cl$_6$.
    The ion Tl$_2$Cl3−9, where thallium atoms are octahedrally coordinated with three bridging chlorine atoms, has been identified in the caesium salt, Cs$_3$Tl$_2$Cl$_9$.

### Thallium(III) bromide complexes

    Salts of Tl$^{III}$Br−4 and Tl$^{III}$Br3−6 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$^−$.

### Thallium(III) iodide complexes

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