Ferroelectric materials can be classified on the basis of their chemical composition and structure as :
(a) Rochelle salt: The first solid to show ferroelectric properties is Rochelle salt. Rochelle salt is the sodium-potassium salt of tartaric acid (NaKC4H4O6.4H2O). It has the property of being ferroelectric only in the temperature range between -18 C and 23 C, which means it has two transition temperatures.
(b) Dihydrogen phosphates and arsenates: Busch and Scherier discovered ferroelectric properties in KH2PO4 in 1935 which is an example of the dihydrogen phosphate and arsenate of the alkali metals. The spontaneous polarization of this material a function of temperature.
In this case, there is only one Curie temperature = 123 K
(c) Oxygen Octahedron group (Barium Titanate): The best known ferroelectric material is barium titanate (BaTiO3) and it is a representative of the oxygen octahedron group of ferroelectric
material. The reason for this name is that the above the Curie temperature that is at 120 C, BaTiO3 corresponds to the cubic structure.
When the temperature is decreased through the critical temperature of 120 C, the material becomes spontaneously polarized and at the same time the structure changes.
BaTiO3 has two more transition temperatures; one at 5 C, and other at -80 C. There will be a change in the crystal structure of the material associated with there ferroelectric transitions.