Ferroelectrics are spontaneously polarised, but are also piezoelectric, in that their polarisation changes under the influence of a stress.This is because while all ferroelectrics are piezoelectric, not all piezoelectrics are ferroelectric.
This relationship can be viewed as:
Pyroelectrics are materials which typically experience a decrease in polarisation when their temperature is increased. They will not be considered in this TLP but a short aside on pyroelectrics can be found in the Ferroelectric Materials TLP.
The piezoelectric effect in ferroelectrics is very dependent on its atomic structure. Depending on the orientation of a crystal, applying a compressive stress can increase or decrease the polarisation, or sometimes, have no effect at all.
To illustrate this, consider the tetragonal phase of BaTiO3, which is commonly seen at room temperature. It possesses a spontaneous polarisation, formed by the dipole moment in each unit cell. To make it simple, we will only consider a single unit cell first. Consider the unit cell of BaTiO3:
Below 120°C this unit cell becomes tetragonal, and gains a spontaneous dipole moment:
If the material is compressed along the x-axis, the important charged ions move further from their original positions, giving a higher dipole moment.
Compressed along the z-axis, the dipole moment decreases as the ions move towards their original position.
This shows how polarisation can easily arise on the atomic level.
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