Calcium fluoride

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.

Calcium fluoride
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.029.262
EC Number
  • 232-188-7
RTECS number
  • EW1760000
UNII
  • InChI=1S/Ca.2FH/h;2*1H/q+2;;/p-2 checkY
    Key: WUKWITHWXAAZEY-UHFFFAOYSA-L checkY
  • InChI=1/Ca.2FH/h;2*1H/q+2;;/p-2
    Key: WUKWITHWXAAZEY-NUQVWONBAZ
  • [Ca+2].[F-].[F-]
  • F[Ca]F
Properties
CaF2
Molar mass 78.075 g·mol−1
Appearance White crystalline solid (single crystals are transparent)
Density 3.18 g/cm3
Melting point 1,418 °C (2,584 °F; 1,691 K)
Boiling point 2,533 °C (4,591 °F; 2,806 K)
0.015 g/L (18 °C)
0.016 g/L (20 °C)
3.9 × 10−11 [1]
Solubility insoluble in acetone
slightly soluble in acid
-28.0·10−6 cm3/mol
1.4338
Structure
cubic crystal system, cF12[2]
Fm3m, #225
a = 5.451 Å, b = 5.451 Å, c = 5.451 Å
α = 90°, β = 90°, γ = 90°
Ca, 8, cubic
F, 4, tetrahedral
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 Reacts with concentrated sulfuric acid to produce hydrofluoric acid
NFPA 704 (fire diamond)
NFPA 704 four-colored diamond
0
0
0
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
>5000 mg/kg (oral, guinea pig)
4250 mg/kg (oral, rat)[3]
Safety data sheet (SDS) ICSC 1323
Related compounds
Other anions
 Calcium chloride
Calcium bromide
Calcium iodide
Other cations
 Beryllium fluoride
Magnesium fluoride
Strontium fluoride
Barium fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Chemical structure
Main article: Fluorite structure

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

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.[4]

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.[5] 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 CaF2 molecule is not linear like MgF2, but bent with a bond angle of approximately 145°; the strontium and barium dihalides also have a bent geometry.[6] It has been proposed that this is due to the fluoride ligands interacting with the electron core[7][8] or the d-subshell[9] 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:[10]

CaCO3 + 2 HF → CaF2 + CO2 + H2O

Applications
Main article: Fluorite

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.[11] Hydrogen fluoride is liberated from the mineral by the action of concentrated sulfuric acid:[12]

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. It also allows much smaller wavelengths to pass through.

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

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.[10]

See also

References
  1. Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  2. X-ray Diffraction Investigations of CaF2 at High Pressure, L. Gerward, J. S. Olsen, S. Steenstrup, M. Malinowski, S. Åsbrink and A. Waskowska, Journal of Applied Crystallography (1992), 25, 578-581 doi:10.1107/S0021889892004096
  3. "Fluorides (as F)". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  4. Burr, P. A.; Cooper, M. W. D. (2017-09-15). "Importance of elastic finite-size effects: Neutral defects in ionic compounds". Physical Review B. 96 (9): 094107. arXiv:1709.02037. Bibcode:2017PhRvB..96i4107B. doi:10.1103/PhysRevB.96.094107. S2CID 119056949.
  5. G. L. Miessler and D. A. Tarr "Inorganic Chemistry" 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.
  6. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  7. Gillespie, R. J.; Robinson, E. A. (2005). "Models of molecular geometry". Chem. Soc. Rev. 34 (5): 396–407. doi:10.1039/b405359c. PMID 15852152.
  8. Bytheway, I.; Gillespie, R. J.; Tang, T. H.; Bader, R.F (1995). "Core Distortions and Geometries of the Difluorides and Dihydrides of Ca, Sr, and Ba". Inorg. Chem. 34 (9): 2407–2414. doi:10.1021/ic00113a023.
  9. Seijo, Luis; Barandiarán, Zoila; Huzinaga, Sigeru (1991). "Ab initio model potential study of the equilibrium geometry of alkaline earth dihalides: MX2 (M=Mg, Ca, Sr, Ba; X=F, Cl, Br, I)" (PDF). J. Chem. Phys. 94 (5): 3762. Bibcode:1991JChPh..94.3762S. doi:10.1063/1.459748. hdl:10486/7315.
  10. Aigueperse, Jean; Mollard, Paul; Devilliers, Didier; Chemla, Marius; Faron, Robert; Romano, René; Cuer, Jean Pierre (2000). "Fluorine Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a11_307.
  11. Aigueperse, Jean; Mollard, Paul; Devilliers, Didier; Chemla, Marius; Faron, Robert; Romano, Renée; Cuer, Jean Pierre (2005), "Fluorine Compounds, Inorganic", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, p. 307, doi:10.1002/14356007.a11_307.
  12. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
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