Barium fluoride

Barium fluoride
Identifiers
CAS number	7787-32-8 Y
RTECS number	CQ9100000
Properties
Molecular formula	BaF2
Molar mass	175.34 g/mol
Appearance	white cubic crystals
Density	4.893 g/cm3
Melting point	1368 °C
Boiling point	2260 °C
Solubility in water	0.16 g/100 mL (20 °C)
Solubility	soluble in methanol, ethanol
Structure
Crystal structure	Fluorite (cubic), cF12
Space group	Fm3m, No. 225
Hazards
EU Index	056-002-00-7
EU classification	Harmful (Xn)
R-phrases	R20/22
S-phrases	(S2), S28
NFPA 704	0 3 0
Flash point	Non-flammable
LD50	250 mg/kg, oral (rat)
Related compounds
Other anions	Barium chloride Barium bromide Barium iodide
Other cations	Calcium fluoride Strontium fluoride
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Barium fluoride (BaF₂) is a chemical compound of barium and fluorine, also known as barium(II) fluoride. It is a solid which can be a transparent crystal. It occurs in nature as the mineral frankdicksonite .^[1]

Structure

The solid adopts the fluorite structure and at high pressure the PbCl₂ structure.^[2] In the vapor phase the BaF₂ molecule is non-linear with an F-Ba-F angle of approximately 108°. ^[3] This is an exception to VSEPR theory which would predict a linear structure. Ab initio calculations have been cited to propose that contributions from d orbitals in the shell below the valence shell are responsible.^[4] Another proposal is that polarisation of the electron core of the barium atom creates an approximately tetrahedral distribution of charge that interacts with the Ba-F bonds.^[5]

Applications

Barium Fluoride is transparent from the ultraviolet to the infrared, from 150-200 nm to 11-11.5 µm, and can be used as a material to make optical components such as lenses. It is used eg. in windows for infrared spectroscopy, in particular in the field of fuel oil analysis. Its transmittance at 200 nm is relatively low (0.60), but at 500 nm it goes up to 0.96-0.97 and stays at that level until 9 µm, then it starts falling off (0.85 for 10 µm and 0.42 for 12 µm). The refractive index is about 1.46 from 700nm to 5µm ^[6]

Barium fluoride is also a common, very fast (one of the fastest) scintillator for the detection of X-rays, gamma rays or other high energy particles. One of its applications is the detection of 511 keV gamma photons in positron emission tomography; other material used in this application is eg. bismuth germanate. It responds also to alpha and beta particles, but, unlike most scintillators, it glows in ultraviolet light ^[7]. It can be also used for detection of high-energy (10-150 MeV) neutrons, and use pulse shape discrimination techniques to separate them from simultaneously occurring gamma photons.

When heated to 500 °C, it gets corroded by water, but in dry environment it can be used up to 800 °C. Prolonged exposure to moisture degrades transmission in the vacuum UV range. It is less resistant to water than calcium fluoride, but is the most resistant of all the optical fluorides to high-energy radiation, though its far ultraviolet transmittance is lower than theirs. It is quite hard, and very sensitive to thermal shock.

Barium fluoride is used as a preopacifying agent and in enamel and glazing frits production. Its other use is in the production of welding agents (an additive to some fluxes, a component of coatings for welding rods and in welding powders). It is also used in metallurgy, as a molten bath for refining aluminium.

References

1. ^ Radtke A.S., Brown G.E. (1974). "Frankdicksonite, BaF2, a New Mineral from Nevada". American Mineralogist 59: 885–888. 
2. ^ A.F Wells (1984). Structural inorganic chemistry -5th Edition. Oxford: Clarendon Press. ISBN 0-19-855370-6. 
3. ^ Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN 0-7506-3365-4 
4. ^ Seijo, Luis (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)". The Journal of Chemical Physics 94: 3762. doi:10.1063/1.459748. 
5. ^ Bytheway, Ian (1995). "Core Distortions and Geometries of the Difluorides and Dihydrides of Ca, Sr, and Ba". Inorganic Chemistry 34: 2407. doi:10.1021/ic00113a023. 
6. ^ "Crystran Ltd. Optical Component Materials". http://www.crystran.co.uk/products.asp?productid=75. Retrieved 25 November 2008. 
7. ^ Laval, M (1983). "Barium fluoride — Inorganic scintillator for subnanosecond timing". Nuclear Instruments and Methods in Physics Research 206: 169. doi:10.1016/0167-5087(83)91254-1. 

External links

• MSDS at Oxford University