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A ferroelectric crystal exhibits an electric dipole moment even in the absence of an external electric field. In the ferroelectric state the center of positive charge of the crystal does not coincide with the center of negative charge The plot of polarization versus electric field for the ferroelectric state shows a hysteresis loop. A crystal in a normal dielectric state usually does not show significant hysteresis when the electric field is increased and then reversed both slowly

In some crystals the ferroelectric dipole moment is not changed by an electric field of maximum intensity In these crystals, when the temperature is increased a change in spontaneous moment is observed such crystals are called pyroelectric.

Examples : Lithium niobate, LiNbO3 is pyroelectric at room temperature it has a transition temperature (Tc = 1480k) and a high saturation polarization (50 uC/cm²). It can "poled" which means given a remanent polarization, by an electric field applied over 1400k .