Calcium fluoride is the inorganic compound of the elements

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

and fluorine with the formula CaF₂. It is a white insoluble solid. It occurs as the mineral

fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs sca ...

(also called fluorspar), which is often deeply coloured owing to impurities.

Chemical structure

The compound crystallizes in a cubic motif called the fluorite structure. Xtals combined 2 300ppi

Ca²⁺ centres are eight-coordinate, being centered in a cube of eight F⁻ centres. Each F⁻ centre is coordinated to four Ca²⁺ centres in the shape of a tetrahedron. Although perfectly packed crystalline samples are colorless, the mineral is often deeply colored due to the presence of

F-center An F center or Farbe center (from the original German ''Farbzentrum'', where ''Farbe'' means ''color'' and ''zentrum'' means center) is a type of crystallographic defect in which an anionic vacancy in a crystal lattice is occupied by one or more un ...

s. The same crystal structure is found in numerous ionic compounds with formula AB₂, such as CeO₂, cubic ZrO₂, UO₂, ThO₂, and PuO₂. In the corresponding

anti-structure In crystallography, an anti-structure is obtained from a salt structure by exchanging anion and cation positions. For instance, calcium fluoride, CaF2, crystallizes in a cubic motif called the fluorite structure. The same crystal structure is ...

, called the antifluorite structure, anions and cations are swapped, such as Be₂C.

Gas phase

The gas phase is noteworthy for failing the predictions of

VSEPR theory Valence shell electron pair repulsion (VSEPR) theory ( , ), is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm the ...

; the molecule is not linear like , but bent with a

bond angle Bond or bonds may refer to: Common meanings * Bond (finance), a type of debt security * Bail bond, a commercial third-party guarantor of surety bonds in the United States * Chemical bond, the attraction of atoms, ions or molecules to form chemical ...

of approximately 145°; the strontium and barium dihalides also have a bent geometry. It has been proposed that this is due to the fluoride

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...

s interacting with the electron core or the d-subshell of the calcium atom.

Preparation

The mineral

is abundant, widespread, and mainly of interest as a precursor to HF. Thus, little motivation exists for the industrial production of CaF₂. High purity CaF₂ is produced by treating calcium carbonate with hydrofluoric acid: :CaCO₃ + 2 HF → CaF₂ + CO₂ + H₂O

Applications

Naturally occurring CaF₂ is the principal source of hydrogen fluoride, a commodity chemical used to produce a wide range of materials. Calcium fluoride in the

state is of significant commercial importance as a fluoride source. Hydrogen fluoride is liberated from the mineral by the action of concentrated sulfuric acid:Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. . :CaF₂ + H₂SO₄ → CaSO₄(solid) + 2 HF

Others

Calcium fluoride is used to manufacture optical components such as windows and lenses, used in thermal imaging systems, spectroscopy,

telescopes A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

, and excimer lasers (used for photolithography in the form of a fused lense). It is transparent over a broad range from ultraviolet (UV) to infrared (IR) frequencies. Its low refractive index reduces the need for

anti-reflection coating An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the effici ...

s. Its insolubility in water is convenient as well. It also allows much smaller wavelengths to pass through. Doped calcium fluoride, like natural fluorite, exhibits

thermoluminescence Thermoluminescence is a form of luminescence that is exhibited by certain crystalline materials, such as some minerals, when previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light upon he ...

and is used in

thermoluminescent dosimeter A thermoluminescent dosimeter, or TLD, is a type of radiation dosimeter, consisting of a piece of a thermoluminescent crystalline material inside a radiolucent package. When a thermoluminescent crystal is exposed to ionizing radiation, it abso ...

s. It forms when fluorine combines with calcium.

Safety

CaF₂ is classified as "not dangerous", although reacting it with sulfuric acid produces very toxic hydrofluoric acid. With regards to inhalation, the NIOSH-recommended concentration of fluorine-containing dusts is 2.5 mg/m³ in air.

References

External links

NIST webbook
thermochemistry data

* ttp://www.crystran.co.uk/calcium-fluoride-caf2.htm Crystran Material Databr>MSDS
(University of Oxford) {{Fluorides Calcium compounds Crystals Fluorides Fluorite Alkaline earth metal halides Optical materials Fluorite crystal structure