Lithium fluoride

Lithium fluoride is an inorganic compound with the chemical formula LiF. It is a colorless solid, that transitions to white with decreasing crystal size. Although odorless, lithium fluoride has a bitter-saline taste. Its structure is analogous to that of sodium chloride, but it is much less soluble in water. It is mainly used as a component of molten salts.[3] Formation of LiF from the elements releases one of the highest energy per mass of reactants, second only to that of BeO.

Lithium fluoride

__Li+     __ F−
Names
IUPAC name
Lithium fluoride
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.029.229
EC Number
  • 232-152-0
RTECS number
  • OJ6125000
UNII
Properties
LiF
Molar mass 25.939(2) g/mol
Appearance white powder or transparent crystals,
hygroscopic
Density 2.635 g/cm3
Melting point 845 °C (1,553 °F; 1,118 K)
Boiling point 1,676 °C (3,049 °F; 1,949 K)
0.127 g/100 mL (18 °C)
0.134 g/100 mL (25 °C)
Solubility soluble in HF
insoluble in alcohol
−10.1·10−6 cm3/mol
1.3915
Structure
Face-centered cubic
a = 403.51 pm
Linear
Thermochemistry
1.604 J/(g K)
35.73 J/(mol·K)
Std enthalpy of
formation (ΔfH⦵298)
 -616 kJ/mol
Hazards
GHS pictograms
GHS Signal word Danger
GHS hazard statements
 H301, H315, H319, H335[1]
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
3
0
Lethal dose or concentration (LD, LC):
143 mg/kg (oral, rat)[2]
Related compounds
Other anions
 Lithium chloride
Lithium bromide
Lithium iodide
Lithium astatide
Other cations
 Sodium fluoride
Potassium fluoride
Rubidium fluoride
Caesium fluoride
Francium 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

Manufacturing

LiF is prepared from lithium hydroxide or lithium carbonate with hydrogen fluoride.[4] It can be also generated by reacting sulfur hexafluoride with metallic lithium, as in the engine of Mark 50 torpedo, but this pathway is not used industrially due to the high cost of reagents.

Applications

In molten salts

Fluorine is produced by the electrolysis of molten potassium bifluoride. This electrolysis proceeds more efficiently when the electrolyte contains a few percent of LiF, possibly because it facilitates formation of an Li-C-F interface on the carbon electrodes.[3] A useful molten salt, FLiNaK, consists of a mixture of LiF, together with sodium fluoride and potassium fluoride. The primary coolant for the Molten-Salt Reactor Experiment was FLiBe; LiF-BeF2 (66-33 mol%).

Optics

Because of the large band gap for LiF, its crystals are transparent to short wavelength ultraviolet radiation, more so than any other material. LiF is therefore used in specialized UV optics,[5] (See also magnesium fluoride). Lithium fluoride is used also as a diffracting crystal in X-ray spectrometry.

Radiation detectors

It is also used as a means to record ionizing radiation exposure from gamma rays, beta particles, and neutrons (indirectly, using the 6
3Li
 (n,alpha) nuclear reaction) in thermoluminescent dosimeters. 6LiF nanopowder enriched to 96% has been used as the neutron reactive backfill material for microstructured semiconductor neutron detectors (MSND).[6]

Nuclear reactors

Lithium fluoride (highly enriched in the common isotope lithium-7) forms the basic constituent of the preferred fluoride salt mixture used in liquid-fluoride nuclear reactors. Typically lithium fluoride is mixed with beryllium fluoride to form a base solvent (FLiBe), into which fluorides of uranium and thorium are introduced. Lithium fluoride is exceptionally chemically stable and LiF/BeF2 mixtures (FLiBe) have low melting points (360 to 459 °C or 680 to 858 °F) and the best neutronic properties of fluoride salt combinations appropriate for reactor use. MSRE used two different mixtures in the two cooling circuits.

Cathode for PLED and OLEDs

Lithium fluoride is widely used in PLED and OLED as a coupling layer to enhance electron injection. The thickness of the LiF layer is usually around 1 nm. The dielectric constant (or relative permittivity) of LiF is 9.0.[7]

Natural occurrence

Naturally occurring lithium fluoride is known as the extremely rare mineral griceite.[8]

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References
  1. "Lithium fluoride - Product Specification Sheet". Sigma-Aldrich. Merck KGaA. Retrieved 1 Sep 2019.
  2. "Lithium fluoride". Toxnet. NLM. Archived from the original on 12 August 2014. Retrieved 10 Aug 2014.
  3. Aigueperse J, Mollard P, Devilliers D, et al. (2005). "Fluorine Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a11_307. ISBN 9783527303854.
  4. Bellinger SL, Fronk RG, McNeil WJ, et al. (2012). "Improved High Efficiency Stacked Microstructured Neutron Detectors Backfilled With Nanoparticle 6LiF". IEEE Trans. Nucl. Sci. 59 (1): 167–173. doi:10.1109/TNS.2011.2175749.
  5. "Lithium Fluoride (LiF) Optical Material". Crystran 19. 2012.
  6. McGregor DS, Bellinger SL, Shultis JK (2013). "Present status of microstructured semiconductor neutron detectors". Journal of Crystal Growth. 379: 99–110. doi:10.1016/j.jcrysgro.2012.10.061. hdl:2097/16983.
  7. Andeen C, Fontanella J, Schuele D (1970). "Low-Frequency Dielectric Constant of LiF, NaF, NaCl, NaBr, KCl, and KBr by the Method of Substitution". Phys. Rev. B. 2 (12): 5068–73. doi:10.1103/PhysRevB.2.5068.
  8. "Griceite mineral information and data". Mindat.org. Archived from the original on 7 March 2014. Retrieved 22 Jan 2014.
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