Pyroelectricity is the capacity of some materials to generate a voltage when they are subjected to heat or cold. Due to the variation in the temperature, slight changes occur in the position of atoms with in the crystals as a result, the polarization of the crystal changes. This develops a voltage across the crystal. The voltage that develops across the crystal is not stable and when the temperature change remains as such, the voltage ceases due to leakage of current. This may be due to the movement of electrons in the crystals. The term Pyroelectricity was originated from Greek word Pyr meaning fire and the term electricity.
Pyroelectricity is different from Thermoelectricity in terms of the electric charge. In Pyroelectricity, as the temperature changes the Pyroelectric crystal as a whole and a voltage develops across the crystal where as in Thermoelectricity a voltage develops across the crystal when its one side is kept at one temperature while the other end at a different temperature. The electrical and thermal aspects of the Pyroelectric crystal represent the Pyroelectric effect while the kinetic energy aspect electrical aspect represents the piezoelectric property. All the Pyroelectric materials also show the piezoelectric property. But some piezoelectric materials have a crystal symmetry which does not allow the Pyroelectric property. Some Pyroelectric crystals change their crystal property in response to very minute change in temperature level as seen in the crystals used to make the PIR Sensors. In these Passive Infra Red sensors, the passive infrared emissions due to the body heat of human beings generate voltage across the crystals.
The crystals generating Pyroelectricity falls into thirty two classes based on the number of rotational axes and reflection planes. Out of the 32 classes of crystals, 21 classes are considered as non Centro metric which do not have centre symmetry. Out of these 21 classes, 20 classes exhibit Direct Piezoelectric property. Out of the 20 classes, crystals of 10 classes are polar in nature possessing a dipole and exhibits Pyroelectricity .
If the dipole of the crystal is reversed by applying an electrical field, the crystal shows Ferroelectric property. Out of the 32 classes of Pyroelectric crystals, only 10 are polar crystals showing charge separation even in the absence of an electric field. All the polar crystals are Pyroelectric.