Calcium fluoride is the

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

of the elements

calcium Calcium is a chemical element; it has 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 to it ...

and

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

CaF₂. It is a white solid that is practically insoluble in water. 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 scal ...

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

Chemical structure

The compound crystallizes in a cubic motif called the

fluorite structure The fluorite structure refers to a common motif for compounds with the formula MX2. The X ions occupy the eight tetrahedral interstitial sites whereas M ions occupy the regular sites of a face-centered cubic In crystallography, the cubic (or is ...

Ca²⁺ centres are eight-coordinate, being centred 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 color 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 defect, vacancy in a crystal ...

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 f ...

, 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 conceptual model, 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 Gill ...

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

bond angle Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that deter ...

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 with a 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 el ...

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 Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

with

hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colorless, acidic and highly corrosive. A common concentration is 49% (48–52%) but there are also stronger solutions (e.g. 70%) and pure HF has a boiling p ...

: :CaCO₃ + 2 HF → CaF₂ + CO₂ + H₂O

Applications

Naturally occurring CaF₂ is the principal source of

hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...

, 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 Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

: :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,

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

s, and

excimer laser An excimer laser, sometimes more correctly called an exciplex laser, is a form of ultraviolet laser which is commonly used in the production of microelectronic devices, semiconductor based integrated circuits or "chips", eye surgery, and micro ...

s (used for

photolithography Photolithography (also known as optical lithography) is a process used in the manufacturing of integrated circuits. It involves using light to transfer a pattern onto a substrate, typically a silicon wafer. The process begins with a photosensiti ...

in the form of a fused lens). 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 effi ...

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 hea ...

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.

Molecular calcium fluorides

Well-characterized molecular calcium fluorides are clusters are formed by treating CaF₂ with large, multidentate ligands. Some calcium fluorides can serves as reagents for nucleophilic fluoride addition to organic compounds.

Safety

CaF₂ is classified as "not dangerous", although reacting it with

produces

, which is highly corrosive and toxic. With regards to inhalation, the

NIOSH The National Institute for Occupational Safety and Health (NIOSH, ) is the United States federal agency responsible for conducting research and making recommendations for the prevention of work-related injury, illness, disability, and death. It ...

-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 Data
MSDS
(University of Oxford) {{Fluorides Calcium compounds Crystals Fluorides Fluorite Alkaline earth metal halides Optical materials Fluorite crystal structure