|
Point
defects are atomic scale, 'zerodimension' defects in a crystal structure.
They include missing atoms, "vacancies,"
impurity atoms on lattice sites, "substitutional
impurities," impurity atoms in non-lattice
locations, "interstitial
impurities,"
and interstitial atoms from the host lattice, "self-interstitials."
In
ionic solids it is necessary to conserve overall charge neutrality and
point defects must be created in neutral groups. Schottky
defects vacancies on both the negative and
positive ion lattices. In an alkali halide, NaCl, one Na+
and one Cl- vacancy must be created. The sodium vacancy has
an efective negative charge as the ion was positive, and the chlorine vacancy
has an effective positive charge as the ion was negative. If an impurity
atom such as Ca++
is substituted for a Na+
ion in the lattice, this extra positive charge must be compensated by creating
a sodium vacancy. Frenkel
defects are vacancy-interstitial pairs. In
AgCl silver interstitials and silver vacancies form Frenkel pairs and the
group is charge neutral, the vacancy being negative and the interstitial
positive.
Point
defects are favored thermodynamically as they increase the configurational
entropy of the structure. If the energy of vacancy formation is Q eV/atom,
the equilibrium concentration of vacancies at a temperature T is given
by: XV
= exp(-Q/kT),
where k is Boltzmann's constant. Similar expressions hold for the other
point defects. |
|
