Piezoelectric ceramics can be divided into single crystals and polycrystalline ceramics, the latter being composed of a collection of many small crystals. The term ‘piezoelectric ceramics’ usually refers to polycrystalline ceramics only. Single crystal piezoelectric ceramics are simply referred to as ‘piezoelectric crystals’.
According to the rotational symmetry, crystalline materials can be divided into 32 crystal classes:
21 of these classes lack a center of symmetry;
20 of the non-centrosymmetric crystal classes show piezoelectricity;
10 of the piezoelectric crystal classes show pyroelectricity;
pyroelectric material may be ferroelectric.
Pyroelectricity Of the twenty piezoelectric crystal classes, ten classes exhibit spontaneous polarisation: even in the absence of mechanical stress or an electric field, the centres of positive and negative charge do not coincide, giving rise to a built-in electric dipole in each unit cell. The crystal classes with spontaneous polarisation are said to be pyroelectric.
The pyroelectric effect, i.e., the generation of charge due to a change in temperature, is nothing but a manifestation of the temperature coefficient of the polarisation.
Examples: tourmaline, Rochelle salt, PZT. Quartz, being piezoelectric, is not pyroelectric.
Ferroelectricity The direction of polarisation of some pyroelectric materials can be changed by applying a sufficiently large electric field. If this is the case, the material is also said to be ferroelectric.
Examples: Rochelle salt, PZT. Tourmaline is not ferroelectric.
Ferroelectricity implies pyroelectricity, but the converse does not hold. In analogy to ferromagnetic materials, ferroelectric materials are inherently hysteretic, i.e., the polarisation not only depends on the current value of the electric field, but also on its history.
Unlike piezoelectricity and pyroelectricity, knowledge of the crystal class is not sufficient to establish ferroelectricity. To that end dielectric measurements are required.