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CAS Number 7580-67-8
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Linear Formula LiH
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Molecular Weight 7.95
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EC Number 231-484-3
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MDL number MFCD00011074
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PubChem Substance ID 24851992
Lithium hydride is an inorganic compound with the formula LiH. This alkali metal hydride is a colorless solid, although commercial samples are grey.
Characteristic of a salt-like, or ionic, hydride, it has a high melting point
and is not soluble but reactive with all organic and protic solvents; it is
soluble and non-reactive with certain molten salts such as lithium
fluoride, lithium borohydride, and sodium hydride.
Detailed description
With a
molecular mass of slightly less than 8, it is the lightest ionic compound.
Applications
Hydrogen storage and fuel
With a hydrogen content three times that of NaH, LiH
has the highest hydrogen content of any hydride. LiH is periodically of
interest for hydrogen storage, but applications have been thwarted by its
stability to decomposition. Thus removal of H2 requires temperatures above the
700 °C used for its synthesis, such temperatures are expensive to create
and maintain. The compound was once tested as a fuel component in a model
rocket.[7][8]
Precursor to complex metal hydrides
LiH is not usually a hydride-reducing agent except
in the synthesis of hydrides of certain metalloids. For example, silane is produced by the
reaction of lithium hydride and silicon tetrachloride via the Sundermeyer
process:
4
LiH + SiCl4 → 4 LiCl +
SiH4
Lithium hydride is used in the production of a
variety of reagents for organic synthesis, such as lithium aluminium
hydride (LiAlH4) and lithium borohydride (LiBH4). Triethylborane reacts to give superhydride (LiBHEt3).[9]
In nuclear chemistry and physics
LiH is a desirable material for shielding nuclear reactors and can be
fabricated by casting.[10][11]
Lithium deuteride
LiH, especially lithium-7 deuteride, is a good
moderator for nuclear reactors, because deuterium
has a lower neutron absorption cross-section than aneutronic hydrogen,
decreasing neutron absorption in a reactor. Lithium-7 is preferred for a
moderator because it has a lower neutron cross-section and also forms less
tritium under neutron bombardment.[12]
The corresponding lithium-6 deuteride, formula 6Li2H or 6LiD, is the fusion fuel in thermonuclear weapons. In warheads of the Teller-Ulam design,
a fission trigger explosion
heats, compresses and bombards 6LiD
with neutrons to produce tritium in an exothermic reaction. The
deuterium and tritium (both isotopes of hydrogen) then fuse to produce
helium-4, a neutron and 17.59 MeV of energy.
Before the Castle Bravo nuclear test, it was
thought that only the less common lithium-6 isotope would breed tritium when
struck with fast neutrons. The test showed that the more plentiful lithium-7
also does so, albeit by an endothermic reaction.
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