Editing Crystal structure (section)

== Physical properties ==
Twenty of the 32 crystal classes are [[piezoelectric]], and crystals belonging to one of these classes (point groups) display [[piezoelectricity]]. All piezoelectric classes lack [[inversion symmetry]]. Any material develops a [[dielectric]] polarization when an electric field is applied, but a substance that has such a natural charge separation even in the absence of a field is called a polar material. Whether or not a material is polar is determined solely by its crystal structure. Only ten of the 32 point groups are [[polar point group|polar]]. All polar crystals are [[pyroelectric]], so the ten polar crystal classes are sometimes referred to as the pyroelectric classes.

There are a few crystal structures, notably the [[Perovskite (structure)|perovskite structure]], which exhibit [[ferroelectric]] behavior. This is analogous to [[ferromagnetism]], in that, in the absence of an electric field during production, the ferroelectric crystal does not exhibit a polarization. Upon the application of an electric field of sufficient magnitude, the crystal becomes permanently polarized. This polarization can be reversed by a sufficiently large counter-charge, in the same way that a ferromagnet can be reversed. However, although they are called ferroelectrics, the effect is due to the crystal structure (not the presence of a ferrous metal).