Electronic or Atomic Polarization
This involves the separation of the centre of the electron cloud around an atom with respect to the centre of its nucleus under the application of electric field (see (a) in figure below).
Ionic Polarization
This happens in solids with ionic bonding which automatically have dipoles but which get cancelled due to symmetry of the crystals. Here, external field leads to small displacement of ions from their equilibrium positions and hence inducing a net dipole moment (see (b)).
Dipolar or Orientation Polarization
This is primarily due to orientation of molecular dipoles in the direction of applied field which would otherwise be randomly distributed due to thermal randomization (see (c and d)) and finally
Interface or Space Charge Polarization
This involves limited movement of charges resulting in alignment of charge dipoles under applied field. This usually happens at the grain boundaries or any other interface such as electrode-material interface (see (e and f))
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| Figure 4.6 Schematic of mechanisms of polarization |
Atomic polarization is present in all materials by definition and hence any other mechanism would be an addition.
While mathematical treatment of the first three mechanisms is rather straightforward, interface polarization is not easy to quantify.
Qualitatively, we can see that in the above four mechanisms, the masses of the entities to be displaced are different, with mass getting larger from electronic to ionic to dipolar polarization. This has a direct relation with the frequency of the applied field (see figure below). Intuitively, we can mention that heavier the particular entity, more is the time spent in displacing it. As a result, atomic polarization is the fastest and typically persists at frequencies between ~1013-1015 Hz. In contrast, ionic polarization is sluggish and typically occurs at frequencies between ~109-1013 Hz while dipolar polarization involving movement of molecules happens below 109 Hz. Interface or space charge polarization occurs at frequencies below 10 Hz.
For non-magnetic dielectrics, Maxwell’s electromagnetic theory predicts that the dielectric constant obtained from the electronic contribution is also related to the index of refraction as εr = n2 which is true at very high frequencies, above 1012-1013 Hz. Contribution from any other mechanism will be on top of it. So the total dielectric constant for a material would be εr-electronic (= n2) + er-ionic + εr-dipolar.
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| Figure 4.7 Schematic figures between dielectric constant vs frequency showing various mechanisms |
The following table shows the values of εr and n2 for a variety of materials and the dominant polarization processes in them:
| Material |
εr |
n2 |
Dominant mechanisms |
C (Diamond) |
~5.7 |
5.85 |
Electronic |
Ge |
~16 |
16.73 |
Electronic |
NaCl |
~5.9 |
2.37 |
Electronic and Ionic |
Water (H2O) |
~80 |
1.77 |
Electronic, Ionic and Dipolar |
So, you can see that while carbon and germanium being single elemental materials show electronic polarization only and as a result their dielectric constants match well with the values of n2. However, the same is not the case with NaCl or water which have strong contributions of ionic and ionic and dipolar polarization respectively. We will discuss more about these processes in the latter sections.
