Aluminium fluoride

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Aluminium fluoride
Aluminium trifluoride crystal structure
FeF3structure.jpg
Identifiers
CAS number 7784-18-1 YesY, 32287-65-3 (monohydrate), 15098-87-0 (trihydrate)
PubChem 2124
ChemSpider 2039 YesY
ChEBI CHEBI:49464 YesY
RTECS number BD0725000
Jmol-3D images Image 1
Properties
Molecular formula AlF3
Molar mass 83.9767 g/mol (anhydrous)
101.022 g/mol (monohydrate)
138.023 (trihydrate)
Appearance white, crystalline solid
odorless
Density 3.1 g/cm3 (anhydrous)
2.1 g/cm3 (monohydrate)
1.914 g/cm3 (trihydrate)
Melting point 1,291 °C (2,356 °F; 1,564 K) (anhydrous) (sublimes)
Solubility in water 0.56 g/100 mL (0 °C)
0.67 g/100 mL (20 °C)
1.72 g/100 mL (100 °C)
Structure
Crystal structure Rhombohedral, hR24
Space group R-3c, No. 167
Hazards
EU classification No classification according to EU Regulation (EC) No. 1272/2008.
R-phrases -
S-phrases -
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g., sodium chloride Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Aluminium fluoride (AlF3) is an inorganic compound used primarily in the production of aluminium. This colorless solid can be prepared synthetically but also occurs in nature. It is used in the production of aluminium metal.

Production and occurrence[edit]

The majority of aluminium fluoride is produced by treating alumina with hexafluorosilicic acid:

H2SiF6 + Al2O3 → 2 AlF3 + SiO2 + H2O

Alternatively, it is manufactured by thermal decomposition of ammonium hexafluoroaluminate.[1] For small scale laboratory preparations, AlF3 can also be prepared by treating aluminium hydroxide or aluminium metal with HF.

Aluminium fluoride trihydrate is found in nature as the rare mineral rosenbergite.

Structure[edit]

Its structure adopts the rhenium trioxide motif, featuring distorted AlF6 octahedra. Each fluoride is connected to two Al centers. Because of its 3-dimensional polymeric structure, AlF3 has a high melting point. The other trihalides of aluminium in the solid state differ, AlCl3 has a layer structure and AlBr3 and AlI3, are molecular dimers.[2] Also they have low melting points and evaporate readily to give dimers.[3] In the gas phase aluminium fluoride exists as trigonal molecules of D3h symmetry. The Al-F bond lengths of this gaseous molecule are 163 pm.

Like most gaseous metal trifluorides, AlF3 adopts a planar structure upon evaporation.

Applications[edit]

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.[1]

Niche uses[edit]

Together with zirconium fluoride, aluminium fluoride is an ingredient for the production of fluoroaluminate glasses.

It is also used to inhibit fermentation.

It is a sputtering target for preparation of low index films.

Safety[edit]

AlF3 has low toxicity (LD50 600 mg/kg).

References[edit]

  1. ^ a b J. Aigueperse, P. Mollard, D. Devilliers, M. Chemla, R. Faron, R. Romano, J. P. Cuer, “Fluorine Compounds, Inorganic” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.doi:10.1002/14356007.a11_307
  2. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419. 
  3. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.

External links[edit]