Practical Electron Microscopy and Database

Figure 2379 shows the schematic illustration of the different classes of crystal systems and their properties. Even though all ferroelectric materials are pyroelectric, piezoelectric, and non-centrosymmetric, the converse is not true.

Seven crystal systems
32 point groups
No center of symmetry (21 point groups)	Center of symmetry (11 point groups)
Piezoelectric (20 point groups)	Non-piezoelectric (1 point groups)
Polar or pyroelectric (10 point groups)	Non-pyroelectric
Ferroelectric (Only if polarization Is reversible)	Non-ferroelectric

Figure 2379. Schematic illustration of the different classes of crystal systems and their properties.

The comparison between the pyroelectrics and ferroelectrics is:
         i) Below the Curie temperature, both the pyroelectric and ferroelectric substances develop spontaneous polarizations.
         ii) If the temperature is raised back above the Curie temperature, the polarization disappears again for both pyroelectric and ferroelectric substances.
         iii) When a reversed electric field is applied, the spontaneous polarization reverses in the ferroelectric substances, but not in the pyroelectric substances.

In crystals, anisotropy of many properties clearly arises because the arrangement of their atoms varies in different directions. In general, cubic crystals is less anisotropic than monoclinic ones because of their greater symmetry, for instance, with respect to electrical conductivity. Therefore, cubic crystals do not exhibit electrical polarization when the temperature is changed, so that their pyroelectric effect is isotropic.