Ferro-electricity is the property of some dielectric materials to keep a net residual polarization even after the electric field is removed. They tipically present a hysteresis cycle doing this.
- It is said that all ferro-electrics are also pyro-electrics, but why is this?
I know that Pyro-electricity is due to an asymmetry of position of negative and positive ions in the crystal structure, such that when thermally dilatated a net polarization is created along some direction. Pervoskite is one such crystal structure that permits this:
Note: a similar effect is found in Piezo-electrics where it's instead a mechanical stress to force a polarization in certain directions. While there could exist some piezo-electric material not pyro-electric, is not true the opposite. I believe the reason for this is that more modes of deformation are available to mechanical deformation rather than those to thermal deformation, moreover all modes of deformation are coverable by a certain mechanical deformation.
But i don't understand the necessary connection between Ferro-electricity and Pyro-electricity, where respectively electric field and thermal deformation are involved, in particular why does not exist any ferro-electric material which is not pyro-electric?
Any help is appreciated, thanks