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Sodium
Chloride is an alkali halide with an fcc crystal structure. The basis is
two ions, a sodium cation and a chlorine anion. The crystal lattice parameter
is 0.563 nm. The diagram shows both a unit cell with ion locations indicated
(a) and a space filling model (b) of ionic hard spheres.
The
cohesive energy of NaCl is - 7.90 eV, with a Coulomb contribution of -8.85
eV, a repulsive contribution of 1.02 eV, a van der Waals contribution of
- 0.125 eV, and a term due to the zero point energy of 0.075 eV.
Because
of its ionic bonding the material is an electronic insulator at room temperature
and is transparent in the visible spectral region. At elevated temperatures
the lattice becomes an ionic conductor, with the major contribution to
charge transport from positive ion vacancy motion. |
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