In physics, the dielectric material such as Rochelle salt and barium Titan ate with a domain structure containing dipoles (a symmetric distributions of electrical charge) which spontaneously align. If the centre of the gravity of the positive and negative charges in a body does not coincide in the absence of an applied electric field, the substance is called Ferro Electric.
Ferroelectric crystals often show several curie points and domain structure hysteresis much as do ferromagnetic crystal. When the temperature exceeds a certain value called the curie point, the substance losses its Ferro electric properties. Above the curie point the formation of small regions-domains-is disrupted by thermal agitation and the permittivity is reduced to the usual small value. Below the Curie point, the whole crystal becomes spontaneously polarized. Examples of Ferro electric materials are Rochelle salt, Potassium dihydrogen phosphate and its isomorphous salts and Barium Titanate.The Curie point for Rochelle salt is about 24degreeC and this substance has a very narrow temperature range of about 40degreeC within which it is ferroelectric.
The Curie point for Barium Titanate is 130degreeC above approximately 130degreeC the crystal structure of Barium Titanate has cubic unit cell. The centre of the mass of the negative charges and the positive charges, Ba2+ and Ti4+ ion. There is therefore no net polarization and p=0. Above 130degreeC, therefore, the barium titanate crystal exhibits no permanent polarization and is not ferroelectric. However, below 130degreeC, the structure of barium titanate is tetragonal in which the Ti4+ atom is not located at the centre of mass of the negative and positive charges. The crystal posses a finite polarization vector P and is ferroelectric.
An important and technologically useful characteristic of a ferroelectric crystal is its ability to be poled. Above 130degreeC there is no permanent polarization in the crystal. If we apply a temporary field E and let the crystal cool to below 130degreeC, we can induce the spontaneous polarization P to develop along the field direction. In other words, we would define the c axis is the polar axis along which P develops. It is also called the ferroelectric axis. Ferroelectrics often have very large dielectric constants, and thus are often used as the dielectric material in capacitors. They also often have unusually large nonlinear optical coefficients.
There are two main types of ferroelectrics: displacive and order-disorder. The effect in barium titanate, a typical ferroelectric of the 'Displacive Type' is due to a polarization catastrophe, in which, if an ion displaced from equilibrium slightly, the force from the local electric fields due to the ions in the crystal increase faster than the elastic restoring forces. This leads to a symmetrical shift in the equilibrium ion positions and hence to a permanent dipole moment. In an order-disorder ferroelectric, there is a dipole moment in each unit cell, but at high temperatures they are pointing in random directions. On lowering the temperature and moving through the phase transition, the dipoles order, all pointing in the similar direction in a domain.