2 (a), a plane of a cubic crystal like NaCl is shown. 2 External force acting on an ionic crystal with (a) and without (b) inversion symmetry In order to understand why symmetry plays such an important role for piezoelectricity, we can compare the response to an external force of a crystal structure with inversion symmetry and one without.įig. However, not every piezoelectric shows pyro- or ferroelectricity.įig. Simply put: All ferroelectrics in their polar phase are pyro- and thus piezoelectric. For some of the pyroelectric groups, this spontaneous polarization can be reversed with an outer electric field, and those materials are then called ferroelectrics. The former exhibit spontaneous polarization and are called pyroelectric. Those piezoelectric point groups can be further divided into ten polar and ten nonpolar ones. Of all 32 point groups, 21 are noncentrosymmetric, and of those 20 point groups show piezoelectricity (only point group 432 is an exception). As will be discussed in detail, only crystals which do not exhibit inversion symmetry are potentially piezoelectric. Piezoelectricity, as well as pyro- and ferroelectricity, is a material property that is strongly tied to crystal symmetries. If this response leads to a change in polarization as well, the material is called piezoelectric. When an external force is applied to a crystal, we generally expect the positions of the atoms in the lattice to change in response to mechanical stress.
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