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 CaF
2. 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.
Ca
2+ centres are eight-coordinate, being centered in a cube of eight F
− centres. Each F
− centre is coordinated to four Ca
2+ 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 AB
2, 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
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 ...
is abundant, widespread, and mainly of interest as a precursor to
HF. Thus, little motivation exists for the industrial production of CaF
2. High purity CaF
2 is produced by treating
calcium carbonate with
hydrofluoric acid:
:CaCO
3 + 2 HF → CaF
2 + CO
2 + H
2O
Applications
Naturally occurring CaF
2 is the principal source of
hydrogen fluoride, a commodity chemical used to produce a wide range of materials.
Calcium fluoride in the
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 ...
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. .]
:CaF
2 + H
2SO
4 →
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
CaF
2 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/m
3 in air.
[
]
See also
*List of laser types
This is a list of laser types, their operational wavelengths, and their applications. Thousands of kinds of laser are known, but most of them are used only for specialized research.
Overview
Gas lasers
Chemical lasers
Used as directed-ener ...
* Photolithography
*Skeletal fluorosis
Skeletal fluorosis is a bone disease caused by excessive accumulation of fluoride leading to weakened bones. In advanced cases, skeletal fluorosis causes painful damage to bones and joints.
Symptoms
Symptoms are mainly promoted in the bone structu ...
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