Editing Pyroelectricity (section)

== Crystal classes ==
All [[crystal structures]] belong to one of thirty-two [[Crystal system#Crystal classes|crystal classes]] based on the number of [[Rotational symmetry|rotational axes]] and [[mirror plane|reflection planes]] they possess that leave the crystal structure unchanged ([[point groups]]). Of the thirty-two crystal classes, twenty-one are [[non-centrosymmetric]] (not having a [[Fixed points of isometry groups in Euclidean space|centre of symmetry]]). Of these twenty-one, twenty exhibit direct [[piezoelectricity]], the remaining one being the cubic class 432. Ten of these twenty piezoelectric classes are polar, i.e., they possess a spontaneous polarization, having a dipole in their unit cell, and exhibit pyroelectricity. If this dipole can be reversed by the application of an electric field, the material is said to be [[ferroelectric]]. Any dielectric material develops a dielectric [[polarization (electrostatics)]] when an electric field is applied, but a substance which 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 10 of the 32 point groups are polar. All [[polar crystals]] are pyroelectric, so the ten polar crystal classes are sometimes referred to as the pyroelectric classes.

Piezoelectric crystal classes: 1, 2, m, 222, mm2, 4, -4, 422, 4mm, -42m, 3, 32, 3m, 6, -6, 622, 6mm, -62m, 23, -43m

Pyroelectric: 1, 2, m, mm2, 3, 3m, 4, 4mm, 6, 6mm

===Related effects===

Two effects which are closely related to pyroelectricity are [[ferroelectricity]] and [[piezoelectricity]].  Normally materials are very nearly electrically neutral on the macroscopic level. However, the positive and negative charges which make up the material are not necessarily distributed in a symmetric manner. If the sum of charge times distance for all elements of the basic cell does not equal zero the cell will have an electric dipole moment (a vector quantity). The dipole moment per unit volume is defined as the dielectric polarization. If this dipole moment changes with the effect of applied temperature changes, applied electric field, or applied pressure, the material is pyroelectric, ferroelectric, or piezoelectric, respectively.

The ferroelectric effect is exhibited by materials which possess an [[electric polarization]] in the absence of an externally applied electric field such that the polarization can be reversed if the electric field is reversed.  Since all ferroelectric materials exhibit a spontaneous polarization, all ferroelectric materials are also pyroelectric (but not all pyroelectric materials are ferroelectric).

The piezoelectric effect is exhibited by crystals (such as quartz or ceramic) for which an electric voltage across the material appears when pressure is applied. Similar to pyroelectric effect, the phenomenon is due to the asymmetric structure of the crystals that allows ions to move more easily along one axis than the others. As pressure is applied, each side of the crystal takes on an opposite charge, resulting in a voltage drop across the crystal.

Pyroelectricity should not be confused with [[thermoelectricity]]: In a typical demonstration of pyroelectricity, the whole crystal is changed from one temperature to another, and the result is a temporary voltage across the crystal. In a typical demonstration of thermoelectricity, one part of the device is kept at one temperature and the other part at a different temperature, and the result is a ''permanent'' voltage across the device as long as there is a temperature difference. Both effects convert temperature change to electrical potential, but the pyroelectric effect converts temperature change over ''time'' into electrical potential, while the thermoelectric effect converts temperature change with ''position'' into electrical potential.