# Aluminium fluoride

Aluminium fluoride is an inorganic compound with the formula AlF$_3$. It forms hydrates AlF$_3$·xH$_2$O. Anhydrous AlF$_3$ and its hydrates are all colorless solids. Anhydrous AlF$_3$ is used in the production of aluminium. Several occur as minerals.

## Occurrence and production

Aside from anhydrous AlF$_3$, several hydrates are known. With the formula AlF$_3$·xH$_2$O, 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 may also be used make aluminium fluoride.
    H$_2$[SiF$_6$] + Al$_2$O$_3$ + 3 H$_2$O → 2 AlF$_3$ + SiO$_2$ + 4 H$_2$O
Alternatively, it is manufactured by thermal decomposition of ammonium hexafluoroaluminate. For small scale laboratory preparations, AlF$_3$ can also be prepared by treating aluminium hydroxide or aluminium 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)$_2$F.

## Structure

According to X-ray crystallography, anhydrous AlF$_3$ adopts the rhenium trioxide motif, featuring distorted AlF$_6$ octahedra. Each fluoride is connected to two Al centers. Because of its three-dimensional polymeric structure, AlF$_3$ has a high melting point. The other trihalides of aluminium in the solid state differ, AlCl$_3$ has a layer structure and AlBr$_3$ and AlI$_3$, 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, it lowers the melting point to below 1000 °C and increases the conductivity of the solution. It is into this molten salt that aluminium oxide is dissolved and then electrolyzed to give bulk Al metal.
Aluminium 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 adenosine triphosphate and guanosine triphosphate control most of the reactions involved in metabolism, growth and differentiation. The observation that aluminium fluoride can bind to and activate heterotrimeric G proteins 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, including the role of GTPase-activating proteins.

### 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 it is used as a low-index optical thin film, particularly when far UV transparency is required. Its deposition by physical vapor deposition, particularly by evaporation, is favorable.

## Safety

The reported oral animal lethal dose (LD$_{50}$) of aluminium fluoride is 100 mg/kg. Repeated or prolonged inhalation exposure may cause asthma, and may have effects on the bone and nervous system, resulting in bone alterations (fluorosis), and nervous system impairment.
Many of the neurotoxic effects of fluoride are due to the formation of aluminium fluoride complexes, which mimic the chemical structure of a phosphate and influence the activity of ATP phosphohydrolases and phospholipase D. Only micromolar concentrations of aluminium are needed to form aluminium fluoride.
Human exposure to aluminium fluoride can occur in an industrial setting, such as emissions from aluminium 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 aluminium source; sources of human exposure to aluminium include drinking water, tea, food residues, infant formula, aluminium-containing antacids or medications, deodorants, cosmetics, and glassware. Fluoridation chemicals may also contain aluminium fluoride. Data on the potential neurotoxic effects of chronic exposure to the aluminium species existing in water are limited.