18.2: Polarization

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A pyroelectric material possesses a spontaneous dipole moment, interpreted via the ionic positions. This dipole moment, when normalised by the amount of material present, yields a polarisation. Whether a given sample possesses a net dipole moment depends on domain configurations, which in turn depend on sample history. This polarisation will change when a stress is applied to the material, as pyroelectrics are a sub-set of piezoelectrics. But it will not reverse under the application of an electric field because it will breakdown first, i.e. the coercive field exceeds the breakdown field. This is only true for a pyroelectric material which is not ferroelectric, whereas if it is ferroelectric, the coercive field is smaller than the breakdown field. In other words, ferroelectrics are a subset of pyroelectrics. Below we compare a ferroelectric with a pyroelectric that is not ferroelectric.

Whether or not a material is pyroelectric or ferroelectric depends upon whether the point group it belongs to is polar, i.e. whether there is at least one direction along which no point group symmetry element forces both sides of the crystal to be the same. The polar point groups are:

1, 2, m, mm2, 3, 3m, 4, 4mm, 6, 6mm. (Point groups will not be covered in this TLP.)

Note that in some of these point groups, e.g. class 4, the polar axis is unique.