Lithium fluoride structure, properties, obtaining, uses

The lithium fluoride it is an inorganic solid whose chemical formula is LiF. It is made up of Li ions⁺ and F^- which are linked through an ionic bond. It is found in small amounts in various minerals, especially silicates such as lepidolite, in sea water and in many mineral wells.

It has been widely used in optical devices due to its transparency in a wide range of wavelengths, from the infrared (IR) spectrum to the ultraviolet UV, through the visible.

It has also been used in devices to detect dangerous radiation in jobs where people are exposed to them for a short time. In addition, it is used as a material to melt aluminum or to make glasses for lenses or glasses and in the manufacture of ceramics..

Serves as a coating material for lithium ion battery components and to prevent the initial loss of charge from the batteries.

Article index

• 1 Structure
• 2 Nomenclature
• 3 Properties
  • 3.1 Physical state
  • 3.2 Molecular weight
  • 3.3 Melting point
  • 3.4 Boiling point
  • 3.5 Density
  • 3.6 Refractive index
  • 3.7 Solubility
  • 3.8 Other properties
• 4 Collection and location
• 5 Uses
  • 5.1 In optical applications
  • 5.2 In ionizing or dangerous radiation detectors
  • 5.3 As a material to preliterate lithium batteries cathode
  • 5.4 In various uses
• 6 References

Structure

Lithium fluoride is an ionic compound, that is, formed by the union of the Li cation⁺ and the anion F^-. The force that holds them together is electrostatic and is called the ionic bond..

When lithium combines, it gives up an electron to fluorine, leaving both in a more stable form than the initial one, as explained below.

The element lithium has the following electronic configuration: [He] 1s^two 2s¹ and when transferring an electron the electronic structure looks like this: [He] 1s^two which is much more stable.

The element fluorine whose electronic configuration is: [Ne] 1s^two 2s^two 2 P⁵, when accepting the electron it remains of the form [Ne] 1s^two 2s^two 2 P⁶, more stable.

Nomenclature

- Lithium fluoride

- Fluorolithium

- Lithium monofluoride

Properties

Physical state

White solid, which crystallizes in cubic structure, like sodium chloride NaCl.

Molecular weight

26 g / mol

Melting point

848.2 ºC

Boiling point

1673 ºC, although it volatilizes at 1100-1200 ºC

Density

2,640 g / cm³

Refractive index

1.3915

Solubility

Slightly soluble in water: 0.27 g / 100 g of water at 18 ºC; 0.134 g / 100 g at 25 ° C. Soluble in acid medium. Insoluble in alcohol.

Other properties

Its vapors present dimeric species (LiF)_two and trimeric (LiF)₃. With hydrofluoric acid HF forms lithium bifluoride LiHF_two; with lithium hydroxide forms a double salt LiF. LiOH.

Collection and location

Lithium fluoride LiF can be obtained by the reaction between hydrofluoric acid HF and lithium hydroxide LiOH or lithium carbonate Li_twoCO₃.

However, it is present in small amounts in certain minerals such as lepidolite and in sea water..

Applications

In optical applications

LiF is used in the form of compact crystals in infrared (IR) spectrophotometers due to the excellent dispersion they present in the wavelength range between 4000 and 1600 cm^-1.

Large crystals of LiF are obtained from saturated solutions of this salt. Can replace natural fluorite crystals in various kinds of optical apparatus.

Large, pure crystals are used in optical systems for ultraviolet (UV), visible and IR light and in X-ray monochromators (0.03-0.38 nm).

It is also used as an optical coating material for the UV region due to its broad optical band, greater than that of other metal fluorides..

Its transparency in the far UV (90-200 nm) make it ideal as a protective coating on aluminum (Al) mirrors. LiF / Al mirrors are used in optical telescope systems for applications in space.

These coatings are achieved by physical vapor deposition and layer deposition at the atomic level..

In ionizing or dangerous radiation detectors

Lithium fluoride has been widely used in thermoluminescent detectors for photon, neutron and β (beta) particle radiation..

Thermoluminescent detectors save the energy of radiation when they are exposed to it. Later, when heated, they release the stored energy in the form of light..

For this application the LiF is generally doped with magnesium (Mg) and titanium (Ti) impurities. These impurities generate certain energy levels that act as holes where the electrons released by radiation are trapped. When the material is then heated, these electrons return to their original energy state, emitting light.

The intensity of the light emitted depends directly on the energy absorbed by the material.

Thermoluminescent LiF detectors have been successfully tested to measure complex fields of radiation, such as those present in the Large Hadron Collider, or LHC. Large Hadron Collider), located in the European Organization for Nuclear Research, known as CERN (for its acronym from the French Conseil Européen pour la Recherche Nucléaire).

The radiation in the experiments carried out in this research center present hadrons, neutrons and electrons / positrons, among other types of subatomic particles, all of which can be detected with LiF.

As a material to preliterate the cathode of lithium batteries

LiF has been successfully tested in the form of nanocomposites with cobalt (Co) and iron (Fe) as materials for prelithiation. prelithiation) of the cathode material of lithium ion batteries.

During the first charge cycle or formation stage of a lithium ion battery, the organic electrolyte decomposes to form a solid phase on the surface of the anode..

This process consumes lithium from the cathode and reduces energy by 5 to 20% of the total capacity of the lithium ion battery..

For this reason, the electrochemical prelitiation of the cathode has been investigated, which generates an electrochemical extraction of lithium from the nanocomposite, which acts as a lithium donor, thus avoiding the consumption of lithium from the cathode.

LiF / Co and LiF / Fe nanocomposites have a high capacity to donate lithium to the cathode, being easy to synthesize, stable under environmental conditions and battery processing.

In various uses

Lithium fluoride is used as a welding flux, especially aluminum, and in coatings for welding rods. Also used in aluminum reduction cells.

It is widely used in the manufacture of glasses (such as lenses) in which the coefficient of expansion decreases. It is also used in the manufacture of ceramics. In addition, it is used in the manufacture of enamels and vitreous varnishes..

LiF is a component of rocket fuels and fuels for certain types of reactors..

LiF is also used in light-emitting diodes or photovoltaic components, for the injection of electrons in internal layers.

References

1. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
2. U.S. National Library of Medicine. (2019). Lithium Fluoride. Recovered from: pubchem.ncbi.nlm.nih.gov.
3. Obryk, B. et al. (2008). The response of different types of TL lithium fluoride detectors to high-energy mixed radiation fields. Radiation Measurements 43 (2008) 1144-1148. Recovered from sciencedirect.com.
4. Sun, Y. et al. (2016). In Situ Chemical Synthesis of Lithium Fluoride / Metal Nanocomposite for High Capacity Prelithiation of Cathodes. Nano Letters 2016, 16, 2, 1497-1501. Recovered from pubs.acs.org.
5. Hennessy, J. and Nikzad, S. (2018). Atomic Layer Deposition of Lithium Fluoride Optical Coatings for the Ultraviolet. Inorganics 2018, 6, 46. Recovered from mdpi.com.