18.7: Effect of Structure on the Dielectric Constant

We have already seen that the more available polarisation mechanisms a material possesses, the larger its dielectric constant will be. For example, materials with permanent dipoles have larger dielectric constants than similar, non-polar materials.

In addition, the more easily the various polarisation mechanisms can act, the larger the dielectric constant will be. For example, among polymers, the more mobile the chains are (i.e. the lower the degree of crystallinity ) the higher the dielectric constant will be.

For polar structures, the magnitude of the dipole also affects the magnitude of polarisation achievable, and hence the dielectric constant. Crystals with non-centrosymmetric structures such as barium titanate have especially large spontaneous polarisations and so correspondingly large dielectric constants. Conversely, a polar gas tends to have smaller dipoles, and its low density also means there is less to polarise, therefore polar gases have lower dielectric constants than polar solids or liquids. The density argument also applies for non-polar gases when compared with non-polar solids or liquids.

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