MAGNESIUM FLUORIDE: STRUCTURE, PROPERTIES, SYNTHESIS, USES - CHEMISTRY - 2024

The magnesium fluoride is an inorganic salt having the chemical formula colorless MgF₂. It is found in nature as the mineral sellaite. It has a very high melting point and is very poorly soluble in water. It is relatively inert, because, for example, its reaction with sulfuric acid is slow and incomplete and it resists hydrolysis with hydrofluoric acid (HF) up to 750ºC.

It is a compound little affected by high energy radiation. In addition, it has a low refractive index, high corrosion resistance, good thermal stability, significant hardness, and excellent visible, UV (ultraviolet) and IR (infrared) light transmission properties.

These properties make it have an excellent performance in the optical field and, furthermore, make it a useful material as a catalyst support, coating element, anti-reflective lenses and windows for infrared transmission, among other applications.

Structure

The crystalline structure of chemically prepared magnesium fluoride is of the same type as that of the natural mineral sellaite. It crystallizes in the dipyramidal class of the tetragonal system.

Magnesium ions (Mg2 +) are located in a centered tetragonal lattice-space, while fluoride ions (F-) are found in the same plane as and associated with their Mg2 + neighbors, grouped in pairs with each other. The distance between the Mg2 + and F- ions is 2.07 Å (angstroms) (2.07 × 10-10m).

His crystal coordination is 6: 3. This means that each Mg2 + ion is surrounded by 6 F- ions and each F- ion, in turn, is surrounded by 3 Mg2 + 5 ions.

The structure is very similar to that of the mineral rutile, which is the natural form of titanium dioxide (TiO2), with which it has several crystallographic properties in common.

During its production, magnesium fluoride does not precipitate as an amorphous solid, since the Mg2 + and F- ions do not tend to form polymeric complexes in solution.

Properties

It is interesting to note that magnesium fluoride is a birefringent material. This is an optical property that allows an incident light ray to be split into two separate rays that propagate at different speeds and wavelengths.

Some of its properties are presented in Table 1.

Table 1. Physical and chemical properties of magnesium fluoride.

Synthesis and preparation

It can be prepared in various ways, including the following:

1-Through the reaction between magnesium oxide (MgO) or magnesium carbonate (MgCO3) with hydrofluoric acid (HF) 2:

MgO + 2 HF MgF2 + H2O

MgCO3 + 2 HF MgF2 + CO2 + H2O

2-By reaction between magnesium carbonate and ammonium bifluoride (NH4HF2), both in solid state, at a temperature between 150 and 400ºC2:

150-400ºC

MgCO3 + NH4HF2 MgF2 + NH3 + CO2 + H2O

3-Heating an aqueous solution of magnesium carbonate and ammonium fluoride (NH4F) in the presence of ammonium hydroxide (NH4OH) at 60ºC 2:

60 ° C, NH4OH

MgCO3 + 3 NH4F NH4MgF3 + (NH4) 2CO3

The resulting precipitate of magnesium ammonium fluoride (NH4MgF3) is then heated at 620 ° C for 4 hours to obtain magnesium fluoride:

620ºC

NH4MgF3 MgF2 + NH3 + HF

4-As a by-product of obtaining beryllium (Be) and uranium (U). The fluoride of the desired element is heated with metallic magnesium in a crucible coated with MgF2 2:

BeF2 + Mg Be + MgF2

5-Reacting magnesium chloride (MgCl2) with ammonium fluoride (NH4F) in aqueous solution at room temperature 3:

25ºC, H2O

MgCl2 + 2 NH4F MgF2 + 2NH4Cl

Since the methods of preparing MgF2 are expensive, there are attempts to obtain it more economically, among which the method of producing it from seawater stands out.

This is characterized by adding a sufficient amount of fluoride ions (F-) to seawater, which has an abundant concentration of magnesium ions (Mg2 +), thus favoring the precipitation of MgF2.

Magnesium fluoride optical crystals are obtained by hot pressing of high-quality MgF2 powder, obtained, for example, by the NH4HF2 method.

There are many techniques for preparing magnesium fluoride materials, such as single crystal growth, sintering (compaction to mold or shaping) without pressure, hot pressing, and microwave sintering.

Applications

Optics

MgF2 crystals are suitable for optical applications because they are transparent from the UV region to the middle IR region 2.10.

As an inert film, it is used to alter the light transmission properties of optical and electronic materials. One of the main applications is in VUV optics for space exploration technology.

Due to its birefringence property, this material is useful in polarization optics, in windows and prisms of the Excimer Laser (a type of ultraviolet laser used in eye surgery).

It should be noted that the magnesium fluoride used in the manufacture of thin film optical materials must be free of impurities or compounds that are a source of oxide, such as water (H2O), hydroxide ions (OH-), carbonate ions (CO3 =), sulfate ions (SO4 =), and the like 12.

Catalysis or acceleration of reactions

MgF2 has been used successfully as a catalyst support for the reaction of chlorine removal and hydrogen addition in CFCs (chlorofluorocarbons), known aerosol refrigerants and propellants, and responsible for the damage to the ozone layer of the atmosphere.

The resulting compounds, HFCs (hydrofluorocarbons) and HCFCs (hydrochlorofluorocarbons), do not have this harmful effect on the atmosphere 5.

It has also proven useful as a catalyst support for hydrodesulfurization (removal of sulfur) of organic compounds.

Other uses

The materials generated by the intercalation of graphite, fluorine and MgF2 have high electrical conductivity, which is why they have been proposed for use in cathodes and as electroconductive materials.

The eutectic formed by NaF and MgF2 has energy storage properties in the form of latent heat, which is why it has been considered for use in solar energy systems.

In the area of ​​biochemistry, magnesium fluoride, along with other metal fluorides, is used to inhibit phosphoryl transfer reactions in enzymes.

Recently, MgF2 nanoparticles have been successfully tested as drug delivery vectors in diseased cells for the treatment of cancer.

References

1. Buckley, HE and Vernon, WS (1925) XCIV. The crystal structure of magnesium fluoride. Philosophical Magazine Series 6, 49: 293, 945-951.
2. Kirk-Othmer (1994). Encyclopedia of Chemical Technology, Volume 11, Fifth Edition, John Wiley & Sons. ISBN 0-471-52680-0 (v.11).
3. Peng, Minhong; Cao, Weiping; and Song, Jinhong. (2015). Preparation of MgF2 Translucent Ceramic by Hot Pressing Sintering. Journal of Wuhan University of Technology-Mater: Sci. Ed. Vol. 30 No. 4.
4. Непоклонов, И.С. (2011). Magnesium Fluoride. Source: Own work.
5. Wojciechowska, Maria; Zielinski, Michal; and Pietrowski, Mariusz. (2003). MgF2 as a non-conventional catalyst support. Journal of Fluorine Chemistry, 120 (2003) 1-11.
6. Korth Kristalle GmbH. (2019). Magnesium Fluoride (MgF2). Retrieved 2019-07-12 at: korth.de
7. Sevonkaev, Igor and Matijevic, Egon. (2009). Formation of Magnesium Fluoride Particles of Different Morphologies. Langmuir 2009, 25 (18), 10534-10539.
8. Непоклонов, И.С. (2013). Magnesium Fluoride. Source: Own work.
9. Tao Qin, Peng Zhang and Weiwei Qin. (2017). A novel method to synthesize low-cost magnesium fluoride spheres from seawater. Ceramics International 43 (2017) 14481-14483.
10. Ullmann's Encyclopedia of Industrial Chemistry (1996) Fifth Edition. Volume A11. VCH Verlagsgesellschaft mbH. New York. ISBN 0-89573-161-4.
11. NASA (2013). Engineers inspecting the Hubble Space Telescope's Primary Mirror 8109563. Source: mix.msfc.nasa.gov