Barium fluoride
Encyclopedia
Barium fluoride is a chemical compound of barium
and fluorine
. It is a solid which can be a transparent crystal. It occurs in nature as the mineral frankdicksonite
.
structure and at high pressure the PbCl2
structure. In the vapor phase the BaF2 molecule is non-linear with an F-Ba-F angle of approximately 108°. 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. 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.
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 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 700 nm to 5 µm
Barium fluoride is also a common, very fast (one of the fastest) scintillator
for the detection of X-ray
s, gamma ray
s or other high energy particles. One of its applications is the detection of 511 keV
gamma photons in positron emission tomography
; other materials used in this application include bismuth germanate
. It responds also to alpha and beta particles, but, unlike most scintillators, it does not glow in ultraviolet light. It can be also used for detection of high-energy (10–150 MeV) neutron
s, 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, very sensitive to thermal shock
and fractures quite easily.
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
.
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
and fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
. It is a solid which can be a transparent crystal. It occurs in nature as the mineral frankdicksonite
Frankdicksonite
Frankdicksonite is a halide mineral with the chemical formula BaF2 which corresponds to the chemical compound barium fluoride. It occurs in the Carlin gold deposit of Eureka County, Nevada as cubic crystals sized between 0.1 and 4 mm, and is of hydrothermal origin. Its only associated mineral is...
.
Structure
The solid adopts the fluoriteCalcium fluoride
Calcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . It is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight fluoride...
structure and at high pressure the PbCl2
Lead(II) chloride
Lead chloride is an inorganic compound which is a white solid under ambient conditions. It is poorly soluble in water. Lead chloride is one of the most important lead-based reagents...
structure. In the vapor phase the BaF2 molecule is non-linear with an F-Ba-F angle of approximately 108°. This is an exception to VSEPR theory
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
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. 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.
Applications
Barium fluoride is transparent from the ultravioletUltraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
to the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, 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 in windows for infrared spectroscopy
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
, in particular in the field of fuel oil analysis. Its transmittance
Transmittance
In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, which is the fraction of radiation absorbed by a sample at a specified wavelength...
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 700 nm to 5 µm
Barium fluoride is also a common, very fast (one of the fastest) scintillator
Scintillator
A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...
for the detection of X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s, gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s or other high energy particles. One of its applications is the detection of 511 keV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
gamma photons in positron emission tomography
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
; other materials used in this application include bismuth germanate
Bismuth germanate
Bismuth germanium oxide is an inorganic chemical compound with main use as a scintillator. It forms cubic crystals....
. It responds also to alpha and beta particles, but, unlike most scintillators, it does not glow in ultraviolet light. It can be also used for detection of high-energy (10–150 MeV) neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s, 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
Calcium fluoride
Calcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . It is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight 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, very sensitive to thermal shock
Thermal shock
Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients...
and fractures quite easily.
Barium fluoride is used as a preopacifying
Opacifier
An opacifier is a substance added to a material in order to make the ensuing system opaque. An example of a chemical opacifier is tin dioxide , which is used to opacify ceramic glazes and milk glass; bone ash is also used....
agent and in enamel
Vitreous enamel
Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...
and glazing frits production. Its other use is in the production of welding
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
agents (an additive to some fluxes
Flux (metallurgy)
In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
, a component of coatings for welding rods and in welding powders). It is also used in metallurgy
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
, as a molten bath for refining aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
.