compared to the wavelength of light. It has to be said, that every material has a polarization, because
everything consists of atoms.
Ionic polarizability:
The effect of ionic polarizability only appears in ionic substances. The positive and negative ions are
pushed in different directions by an electric field and local dipole moments are induced.
Orientation (dipolar) polarizability:
It is easy to understand this effect when we think of a gas with permanent dipole moments. In such
a gas at high temperatures and zero field, all the dipoles are oriented randomly. If a field is applied,
the average of the dipoles will show in the direction of the field. The theory is very similar to the
theory of paramagnetism. There the spins get oriented by an external field and at low temperatures
and high fields there will be saturation. If we do the same calculation for the polarization as for the
paramagnetism, we see that the susceptibilityχdiverges at low temperatures:
χ∝1
T
(249)
For gases and liquids this effect is easy to describe, because there the dipole moments can arrange
freely. However, in solids the moments are bound and so another model is needed to describe this
effect here. The theory is based on ion jumps in solid crystals which have defects. In such defects
double charged ions are attracted and trapped. In fig. 128 the double charged ion is marked with a
bold border. Now it could happen, that this ion changes its position with a single charged ion and
what happens is that the polarization changes the direction.
Figure 128: A solid crystal with a defect, where a double charged ion is attracted and trapped. When
this ion swaps the position with another ion, the polarization will change the direction
Space charge polarizability: