The thallium halides include monohalide

Halide

A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...

s, where thallium

Thallium

Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...

 has oxidation state

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 +1, trihalides where thallium generally has oxidation state

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 +3 and some intermediate halides with mixed +1 and +3 oxidation states

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

.

Monohalides

The monohalides all contain thallium with oxidation state

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 +1. Parallels can be drawn between the thallium(I) halides and their corresponding silver

Silver

Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

 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

Thallium(I) fluoride

Thallium fluoride is the chemical compound composed of thallium and fluorine with the formula TlF. It consists of hard white orthorhombic crystals which are slightly deliquescent in humid air but revert to the anhydrous form in dry air...

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 40Gpa.

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

Thallium(I) chloride

Thallium chloride is the chemical compound with the formula TlCl. This colourless solid is an intermediate in the isolation of thallium from its ores. Typically, an acidic solution of thallium sulfate is treated with hydrochloric acid to precipitate insoluble thallium chloride...

TlCl is a light sensitive, white crystalline solid, mp 430 °C. The crystal structure is the same as CsCl

Caesium chloride

Caesium chloride is the inorganic compound with the formula CsCl. This colorless solid is an important source of caesium ions in a variety of applications. Its crystal structure forms a major structural type where each caesium ion is coordinated by 8 chlorine ions...

.

Thallium(I) bromide

Thallium(I) bromide

Thallium bromide , a chemical compound, available in an ultra-pure state is a compound semiconductor; used in room temperature X- and gamma-ray detectors and blue sensitive photodetectors; used as a real-time x-ray image sensor; also used as a standard for elemental thallium.The crystalline...

TlBr is a light sensitive, pale yellow crystalline solid, mp 460 °C. The crystal structure is the same as CsCl

Caesium chloride

Caesium chloride is the inorganic compound with the formula CsCl. This colorless solid is an important source of caesium ions in a variety of applications. Its crystal structure forms a major structural type where each caesium ion is coordinated by 8 chlorine ions...

.

Thallium(I) iodide

Thallium(I) iodide

Thallium iodide is a chemical compound of formula TlI. It is unusual in being one of the few water-insoluble metal iodides, along with AgI, PbI2 and HgI2.-Chemistry:...

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

Caesium chloride

Caesium chloride is the inorganic compound with the formula CsCl. This colorless solid is an important source of caesium ions in a variety of applications. Its crystal structure forms a major structural type where each caesium ion is coordinated by 8 chlorine ions...

.

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 focussing of infrared radiation . The materials were originally developed and produced by Carl Zeiss 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_0.4I_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.

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

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 +3 but is a thallium(I) compound and contains the linear triiodide (I₃⁻)

Triiodide

In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, ...

 ion.

Thallium(III) fluoride

TlF₃ is a white crystalline solid , mp 550 °C. The crystal structure is the same as YF₃

Yttrium(III) fluoride

Yttrium fluoride is an inorganic chemical compound with the chemical formula YF3. It is not known naturally in 'pure' form. The fluoride minerals containing essential yttrium include tveitite- 6Ca6Ca6F42 and gagarinite- NaCaY6...

  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

Bromine trifluoride

Bromine trifluoride is an interhalogen compound with the formula BrF3. This toxic, colourless, and corrosive liquid is soluble in sulfuric acid but explodes on contact with water and organic compounds. It is a powerful fluorinating agent and an ionizing inorganic solvent...

 or SF₄

Sulfur tetrafluoride

Sulfur tetrafluoride is the chemical compound with the formula SF4. This species exists as a gas at standard conditions. It is a corrosive species that releases dangerous HF upon exposure to water or moisture...

 at 300 °C.

Thallium(III) chloride

TlCl₃ has a distorted Cr(III) chloride

Chromium(III) chloride

Chromium chloride is a violet coloured solid with the formula CrCl3. The most common form of CrCl3 sold commercially is a dark green hexahydrate with the formula [CrCl24]Cl.2H2O. Two other hydrates are known, pale green [CrCl5]Cl2.H2O and violet [Cr6]Cl3...

 structure like AlCl₃

Aluminium chloride

Aluminium chloride is the main compound of aluminium and chlorine. It is white, but samples are often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large...

 and InCl₃. Solid TlCl₃ is unstable and disproportionates

Disproportionation

Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....

  at 40 °C, losing chlorine to give TlCl

Thallium(I) chloride

Thallium chloride is the chemical compound with the formula TlCl. This colourless solid is an intermediate in the isolation of thallium from its ores. Typically, an acidic solution of thallium sulfate is treated with hydrochloric acid to precipitate insoluble thallium chloride...

. It can be prepared in CH₃CN

Acetonitrile

Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...

 by treating a solution of TlCl

Thallium(I) chloride

Thallium chloride is the chemical compound with the formula TlCl. This colourless solid is an intermediate in the isolation of thallium from its ores. Typically, an acidic solution of thallium sulfate is treated with hydrochloric acid to precipitate insoluble thallium chloride...

 with Cl₂ gas.

Thallium(III) bromide

This unstable compound disproportionates

Disproportionation

Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....

 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

Thallium triiodide

Thallium triiodide is a chemical compound of thallium and iodine with formula TlI3. Unlike the other thallium trihalides, which contain thallium, TlI3 is a thallium compound and contains the triiodide ion, I3−....

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^-.

TlCl₂

This is formulated as Tl^ITl^IIICl₄.

Tl₂Cl₃

This yellow compound is formulated Tl^I₃ Tl^IIICl₆.

Tl₂Br₃

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

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

Scintillator

A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...

.

Thallium(III) fluoride complexes

The salts NaTlF₄ and Na₃TlF₆ do not contain discrete tetrahedral

Tetrahedral molecular geometry

In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...

 and octahedral

Octahedral molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...

 anions. The structure of NaTlF₄ is the same as fluorite (CaF₂)

Calcium fluoride

Calcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . It is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight fluoride...

 with Na^I and Tl^III atoms occupying the 8 coordinate Ca^II sites. Na₃TlF₆ has the same structure as cryolite

Cryolite

Cryolite is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, depleted by 1987....

, Na₃AlF₆. In this the thallium atoms are octahedrally

Octahedral molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...

 coordinated. Both compounds are usually considered to be mixed salts of Na⁺ and Tl³⁺.

Thallium(III) chloride complexes

Salts of tetrahedral

Tetrahedral molecular geometry

In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...

 TlCl₄^- and octahedral

Octahedral molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...

 TlCl₆^3-are known with various cations.

Salts containing TlCl₅^2- with a square pyramidal structure are known. Interestingly some salts, that nominally contain TlCl₅^2- actually contain the dimeric anion Tl₂Cl₁₀^4-, long chain anions where Tl^III is 6 coordinate and the octahedral

Octahedral molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...

 units are linked by bridging chlorine atoms, or mixed salts of Tl^IIICl₄ and Tl^IIICl₆.

The ion Tl₂Cl₉^3- where thallium atoms are octahedrally

Octahedral molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...

 coordinated with three bridging chlorine atoms has been identified in the Caesium salt, Cs₃Tl₂Cl₉.

Thallium(III) bromide complexes

Salts of Tl^IIIBr₄^- and Tl^IIIBr₆^3- are known with various cations.

The TlBr₅^2- anion has been characterised in a number of salts and is trigonal bipyramidal

Trigonal bipyramid molecular geometry

In chemistry a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular dipyramid...

. Some other salts that nominally contain TlBr₅^2- are mixed salts containing TlBr₄^- and Br^-.

Thallium(III) iodide complexes

Salts of Tl^IIII₄^- are known. The Tl^III anion is stable even though the triiodide is a thallium(I) compound.