Ferroelectricity

One calls ferroelectricity the property according to which a material has an electric polarization in a spontaneous state, polarization which can be reversed by the application of a external Electric field.

One can explain that simply by evoking the shift of the Barycentre S of the positive and negative loads.

History

The ferroelectricity was known a long time only in the salt of Seignette, a salt of chemical composition and complex crystallographic structure. This complexity slowed down research and let think that the ferroelectricity was a completely exotic property requiring quite particular conditions (hydrogen bonds in particular). Moreover then, this property did not find any interest practical.

A major jump in the study of ferroelectric was the discovery with beginning of the year 1950 of the ferroelectric oxides of structure perovskite: BaTiO3, PbTiO3, etc These materials simpler allowed the development of theory of the ferroelectricity.

Ferroelectric transitions from phase

Usually one observes a fast variation of the properties Diélectrique S (for example polarization) at the temperature of Tc Curie (or temperature of Transition from phase Ttr). If it is continuous, it indicates a transition from the second order according to the theory of Landau, because the degree of order continuously tightens towards Zero when the Température increases.

One generally distinguishes two kinds of ferroelectric transitions:

  • the order-disorder transition: generally, in the paraelectric phase the meshs of the Cristal have one dipole moment without privileged orientation because the permanent dipoles are perfectly disordered; macroscopic polarization is thus null. On the contrary in the ferroelectric phase the dipole moments of the various meshs present an orientational order.

  • the displacive transition (of English displace which wants to say to move): it is the typical transition from the Perovskite S (like BaTiO3, PZT (PbZr1-xTixO3), BST (Ba1-xSrxTiO3)…). The dipole moment is non-existent in the paraelectric phase, even from the microscopic point of view, generally for reasons of crystalline symmetry (the material is then in its cubic phase). It appears at the time of the transition because the displacement of the Atome S creates dipoles directed according to the axis of motion.

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