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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.

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:

                                            \text{Pervoskite structure under pyro-electric effect}

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

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  • $\begingroup$ They are linked through the reduced symmetry of the crystals (non-cubic) leading to those effects. Or, ferro- and pyro-electricity are two sides of the same coin. $\endgroup$
    – Jon Custer
    Jun 18, 2020 at 22:38
  • $\begingroup$ Actually not all pyroelectrics are also ferroelectrics $\endgroup$ Jun 18, 2020 at 22:44

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I think crystal structure (https://en.wikipedia.org/wiki/Pyroelectricity) explains this. Say if the pervoskite (which is pyro with natural polarisation P) red atom jumps from upper half to lower half of gold enclosure and polarisation reverses to downward direction when applying an electrical field, then it is ferroelectric too, otherwise only pyroelectric. So perhaps it is an electric potential energy barrier issue that determines ferroelectricity of a polar crystal which is always pyro.

Note, this assumes stable reversal. High intensity and / or short-lived reversals can happen and if observed can settle your disquiet.

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  • $\begingroup$ Actually my understanding was that all unit cell deform, when stress or thermal or electric field applies. Also from a macroscopic level is impossible to say if polarization happened because of 'reversal' or not, we can just observe if polarization is kept after electric field is removed. But thinking about it, i realized that the reason might be that inclined dipoles align, leaving polarization even after electric field is removed. And this can only happen if dipoles are already present in matter, as piezoelectricity does not necessarly need that, ferroelectric must also be pyroelectric $\endgroup$ Jun 20, 2020 at 21:32
  • $\begingroup$ but i still don't understand why some pyroelectrics (then with pre-existing polarization) are not ferroelectric also $\endgroup$ Jun 20, 2020 at 21:34
  • $\begingroup$ Perhaps my answer was not explicit enough for illustration. Let's for the explanation assume for heuristic purposes that the single crystal cell is electro-statically neutral but as in the pervoskite-like structure you showed, that the red atom is +vely charged, balanced by the rest of the atoms in the single cell. Because of the asymmetry in its position the dipole manifests as shown. Were that atom in the reflection opposite position, the dipole would have reversed, no? If yes, then if it could be moved (energy barrier allowing) there by an external field, then it would be ferroelectric. $\endgroup$
    – MKhomo
    Jun 21, 2020 at 17:59
  • $\begingroup$ And to account for macroscopic manifestation one can assume equidistant red atoms more stable than a mix of closer / further random arrangement in selection of which asymmetric offset to stabilise on after the electric field is removed. Hence a macro polarisation which should also be observerbale across an aggregate multicrystal body each one randomly aligned. $\endgroup$
    – MKhomo
    Jun 21, 2020 at 18:15
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    $\begingroup$ Finally, the energy barrier heuristic is generic to all crystal structures, not only polar ( ferro / pyro / piezo). Imangine if that red atom was relatively so big that to occupy the mirrored state requires changes in interatomic separation of the gold and blue atoms. That makes reversal not so prevalent compared to say small reds that do not perturb the structure of the but jump back and forth freely and thereby show a bright spectral line for that ferro transition. The only thing that keeps them polarised is the stable inter-cell separation I mentioned before. Ergo your ferro-electrics. $\endgroup$
    – MKhomo
    Jun 22, 2020 at 4:05

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