Английская Википедия:Calcium fluoride

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Calcium fluoride is the inorganic compound of the elements calcium and fluorine with the formula CaF₂. It is a white solid that is practically insoluble in water. It occurs as the mineral fluorite (also called fluorspar), which is often deeply coloured owing to impurities.

Chemical structure

Шаблон:Main The compound crystallizes in a cubic motif called the fluorite structure.

Файл:Xtals combined 2 300ppi.png

Unit cell of CaF₂, known as fluorite structure, from two equivalent perspectives. The second origin is often used when visualising point defects entered on the cation.

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-centers. 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, 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; the Шаблон:Chem2 molecule is not linear like Шаблон:Chem2, but bent with a bond angle of approximately 145°; the strontium and barium dihalides also have a bent geometry. It has been proposed that this is due to the fluoride ligands interacting with the electron core or the d-subshell of the calcium atom.

Preparation

The mineral fluorite 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

Шаблон:Main 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 fluorite state is of significant commercial importance as a fluoride source. Hydrogen fluoride is liberated from the mineral by the action of concentrated sulfuric acid:

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, and excimer lasers (used for photolithography 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 coatings. Its insolubility in water is convenient as well.Шаблон:Cn It also allows much smaller wavelengths to pass through.Шаблон:Cn

Doped calcium fluoride, like natural fluorite, exhibits thermoluminescence and is used in thermoluminescent dosimeters. It forms when fluorine combines with calcium.Шаблон:Cn

Safety

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

See also

• List of laser types
• Photolithography
• Skeletal fluorosis

External links

• NIST webbook thermochemistry data
• Charles Townes on the history of lasers
• National Pollutant Inventory - Fluoride and compounds fact sheet
• Crystran Material Data Шаблон:Webarchive
• MSDS Шаблон:Webarchive (University of Oxford)