# Pyroelectricity

### From Online Dictionary of Crystallography

Pyroélectricité (Fr). Pyroelectrizität (Ge). Pyroelectricidad (Sp).

## Definition

Pyroelectricity is the property presented by certain materials that exhibit an electric polarization Pi when a temperature variation δΘ is applied uniformly:

Pi = piT δΘ

where piT is the pyroelectric coefficient at constant stress. Pyroelectric crystals actually have a spontaneous polarization, but the pyroelectric effect can only be observed during a temperature change. If the polarisation can be reversed by the application of an electric field, the crystal is ferroelectric.

If the crystal is also piezoelectric, the polarization due to an applied temperature variation is also partly due to the piezoelectric effect. The coefficient describing the pure pyroelectric effect is the pyroelectric coefficient at constant strain, piS. The two coefficients are related by:

piT = cijkldklnαjn + piS

where the cijkl are the elastic stiffnesses, the dkln the piezoelectric coefficients and the αjn the linear thermal expansion coefficients.

The converse effect is the electrocalorific effect. If a pyroelectric crystal is submitted to an electic field, it will undergo a change of entropy Δσ:

Δσ = pi Ei

and will release or absorb a quantity of heat gien by Θ V Δσ where Θ is the temperature of the specimen and V its volume.

## Pyroelectric point groups

The geometric crystal classes for which the piezoelectric effect is possible are determined by symmetry considerations (see Curie laws). They are the classes of which the symmetry is a subgroup of the symmetry associated with that of the electric field, AM:

1, 2, 3, 4, 6, m, 2mm, 3m, 4mm, 6mm

## History

The appearance of electrostatic charges upon changes of temperature has been observed since ancient times, in particular on tourmaline. It is Sir David Brewster (1781-1788) who coined the term 'pyroelectricity' (Brewster D., 1824, Edinburgh. J. Sci., 1, 208-215, Observations on the pyroelectricity of minerals, translated into German, Poggendorf Ann. Phys., 1824, 2, 297-307, Beobachtungen über die, in den Mineralien, durch Wärme erregte Electricität).