Lithium fluoride

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Lithium fluoride
Lithium fluoride boule
Lithium fluoride
Identifiers
CAS number 7789-24-4 YesY
PubChem 224478
ChemSpider 23007 YesY
EC number 232-152-0
RTECS number OJ6125000
Jmol-3D images Image 1
Properties
Molecular formula LiF
Molar mass 25.939(2) g/mol
Appearance white powder or transparent crystals,
non-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)
Solubility in water 0.27 g/100 mL (18 °C)[1]
0.134 g/100 mL (25 °C)
Solubility soluble in HF
insoluble in alcohol
Refractive index (nD) 1.3915
Structure
Crystal structure Cubic
Lattice constant a = 403.51 pm
Molecular shape Linear
Thermochemistry
Specific
heat capacity
C
1.604 J/(g K)
Std molar
entropy
So298
1.376 J/(g K)
Std enthalpy of
formation
ΔfHo298
-616 kJ/mol
Hazards
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
LD50 143 mg/kg (oral, rat)[2]
Related compounds
Other anions Lithium chloride
Lithium bromide
Lithium iodide
Other cations Sodium fluoride
Potassium fluoride
Rubidium fluoride
Caesium fluoride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

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 releases one of the highest energy per mass of reactants, only second to that of BeO.

Manufacturing[edit]

LiF is prepared from lithium hydroxide and hydrogen fluoride or by dissolving lithium carbonate in excess hydrogen fluoride, evaporating to dryness and heating to red heat.

Applications[edit]

In molten salts[edit]

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 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[edit]

Because of its large band gap, LiF crystals are transparent to short wavelength ultraviolet radiation, more so than any other material. LiF is therefore used in specialized UV optics,[4] (See also magnesium fluoride). Lithium fluoride is used also as crystal in X-ray spectrometry.

Radiation detectors[edit]

It is also used as a means to record ionizing radiation exposure from gamma rays, beta particles, and neutrons (indirectly, using the 6
3
Li
(n,alpha) nuclear reaction) in thermoluminescent dosimeters.

Nuclear reactors[edit]

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 C - 459 C) 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[edit]

Lithium fluoride is widely used in PLED and OLED as a coupling layer to enhance electron injection. The thickness of LiF layer is usually around 1 nm. The dielectric constant (or relative permittivity) of LiF is 9.0[5]

References[edit]

  1. ^ "Lithium fluoride". Retrieved 2006-02-26. 
  2. ^ http://chem.sis.nlm.nih.gov/chemidplus/rn/7789-24-4
  3. ^ a b J. Aigueperse, P. Mollard, D. Devilliers, M. Chemla, R. Faron, R. Romano, J. P. Cuer, “Fluorine Compounds, Inorganic” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. doi:10.1002/14356007.a11_307.
  4. ^ "Crystran Ltd., a manufacturer of infrared and ultraviolet optics". Retrieved 2010-12-28. 
  5. ^ C. Andeen, J. Fontanella,D. Schuel, “Low-Frequency Dielectric Constant of LiF, NaF, NaC1, NaBr, KC1, and KBr by the Method of Substitution”, Physical Review B, 2, 5068-5073 (1970) doi:10.1103/PhysRevB.2.5068.