Ferroelectrics have a transition temperature, called the Curie point (typically between 150 °C and 300 °C), above which the unit cell is centrosymmetric and does not exhibit ferroelectricity. The material is then said to be in a paraelectric phase.
When a ferroelectric crystal cools through its Curie point, it undergoes a so-called ferroelectric distortion. The unit cell loses part of its symmetry and an electric dipole arises, the possible directions of the polar axis within the unit cell depending on the particular ferroelectric distortion.
Upon cooling through the Curie point, throughout the crystal several ferroelectric domains originate. These are regions with a uniform polarisation, separated by domain walls. The particular domain pattern formed is a result of the stresses created at the Curie point, uncompensated surface charges, and physical imperfections.
Crystals which are spontaneously single-domain are rare. Thus, in general, a ferroelectric crystal does not show a macroscopic polarisation. Nevertheless, even without macroscopic polarisation, a ferroelectric single crystal does exhibit piezoelectricity.