Aluminium fluoride refers to inorganic compounds with the formula AlF₃·''x''H₂O. They are all colorless solids. Anhydrous AlF₃ is used in the production of aluminium metal. Several occur as minerals.

Occurrence and production

Aside from anhydrous AlF₃, several hydrates are known. With the formula AlF₃·''x''H₂O, these compounds include monohydrate (''x'' = 1), two polymorphs of the trihydrate (''x'' = 3), a hexahydrate (''x'' = 6), and a nonahydrate (''x'' = 9). The majority of aluminium fluoride is produced by treating alumina with hydrogen fluoride at 700 °C:

Hexafluorosilicic acid Hexafluorosilicic acid is an inorganic compound with the chemical formula . Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless. Hexafluo ...

may also be used make aluminum fluoride. :H₂SiF₆ + Al₂O₃ + 3 H₂O → 2 AlF₃ + SiO₂ + 4 H₂O Alternatively, it is manufactured by thermal decomposition of ammonium hexafluoroaluminate. For small scale laboratory preparations, AlF₃ can also be prepared by treating

aluminium hydroxide Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic an ...

or aluminium metal with hydrogen fluoride. Aluminium fluoride trihydrate is found in nature as the rare mineral rosenbergite. The anhydrous form appears as the relatively recently (as of 2020) recognized mineral óskarssonite. A related, exceedingly rare mineral, is zharchikhite, Al(OH)₂F.

Structure

According to

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

, anhydrous AlF₃ adopts the

rhenium trioxide Rhenium trioxide or rhenium(VI) oxide is an inorganic compound with the formula ReO3. It is a red solid with a metallic lustre that resembles copper in appearance. It is the only stable trioxide of the Group 7 elements ( Mn, Tc, Re). Prepara ...

motif, featuring distorted AlF₆

octahedra In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

. Each fluoride is connected to two Al centers. Because of its three-dimensional polymeric structure, AlF₃ has a high

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...

. The other trihalides of aluminium in the solid state differ, AlCl₃ has a layer structure and AlBr₃ and AlI₃, are molecular dimers. Also they have low melting points and evaporate readily to give dimers. In the gas phase aluminium fluoride exists as trigonal molecules of ''D_3h'' symmetry. The Al–F bond lengths of this gaseous molecule are 163 pm.

Applications

Aluminium fluoride is an important additive for the production of aluminium by electrolysis. Together with

cryolite Cryolite ( Na3 Al F6, sodium hexafluoroaluminate) is an uncommon mineral identified with the once-large deposit at Ivittuut on the west coast of Greenland, mined commercially until 1987. History Cryolite was first described in 1798 by Danish vete ...

, it lowers the melting point to below 1000 °C and increases the conductivity of the

solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Soluti ...

. It is into this molten salt that aluminium oxide is dissolved and then electrolyzed to give bulk Al metal. Aluminum fluoride complexes are used to study the mechanistic aspects of phosphoryl transfer reactions in biology, which are of fundamental importance to cells, as phosphoric acid anhydrides such as ATP and GTP control most of the reactions involved in metabolism, growth and differentiation. The observation that aluminum fluoride can bind to and activate heterotrimeric

G protein G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. Their a ...

s has proven to be useful for the study of G protein activation in vivo, for the elucidation of three-dimensional structures of several GTPases, and for understanding the biochemical mechanism of GTP

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...

, including the role of

GTPase-activating protein GTPase-activating proteins or GTPase-accelerating proteins (GAPs) are a family of regulatory proteins whose members can bind to activated G proteins and stimulate their GTPase activity, with the result of terminating the signaling event. GAPs are ...

Niche uses

Together with zirconium fluoride, aluminium fluoride is an ingredient for the production of fluoroaluminate glasses. It is also used to inhibit fermentation. Like

magnesium fluoride Magnesium fluoride is an inorganic compound with the formula MgF2. The compound is a white crystalline salt and is transparent over a wide range of wavelengths, with commercial uses in optics that are also used in space telescopes. It occurs natur ...

it is used as a low-index optical

thin film A thin film is a layer of material ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...

, particularly when far UV transparency is required. Its deposition by

physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polym ...

, particularly by evaporation, is favorable.

Safety

The reported oral animal lethal dose ( LD₅₀) of aluminum fluoride is 0.1 g/kg. Repeated or prolonged inhalation exposure may cause

asthma Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...

, and may have effects on the bone and nervous system, resulting in bone alterations ( fluorosis), and nervous system impairment. Many of the

neurotoxic Neurotoxicity is a form of toxicity in which a biological, chemical, or physical agent produces an adverse effect on the structure or function of the central and/or peripheral nervous system. It occurs when exposure to a substance – specificall ...

effects of fluoride are due to the formation of aluminum fluoride complexes, which mimic the chemical structure of a

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

and influence the activity of ATP phosphohydrolases and phospholipase D. Only

micromolar Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of sol ...

concentrations of aluminum are needed to form aluminum fluoride. Human exposure to aluminum fluoride can occur in an industrial setting, such as emissions from aluminum reduction processes, or when a person ingests both a fluoride source (e.g., fluoride in drinking water or residue of fluoride-based pesticides) and an aluminum source; sources of human exposure to aluminum include drinking water, tea, food residues, infant formula, aluminum-containing antacids or medications, deodorants, cosmetics, and glassware. Fluoridation chemicals may also contain aluminum fluoride. Data on the potential neurotoxic effects of chronic exposure to the aluminum species existing in water are limited.Aluminum Compounds Review of Toxicological Literature Abridged Final Report
Prepared for National Institute of Environmental Health Sciences. ''NTP.gov Nomination Summary for Aluminum contaminants of drinking water (N20025).'' October 2001

References

External links

MSDS

ToxNet Profile

PubChem
{{Authority control Aluminium compounds Fluorides Metal halides