Ferroelectric Materials

Certain types of dielectric Material (like Rochelle salt and Barium titanate) become spontaneously polarized in the absence of external fields. The self – polarization results from the displacement produce forces on the ions that are greater for small displacements than the elastic restoring forces within the crystal. As a result, the equilibrium position of the ions is such as to give a net polarization in the crystal. Such materials are called ferroelectric. Ferroelectric material often exhibit large susceptibility. They show hysteresis in the polarization produced by external fields and lose their spontaneous polarization above some critical temperature.

Ferroelectric crystals have very large dielectric constants. The high dielectric constant of ferroelectric crystals is useful for storing energy in small-sized capacitors in electrical circuits.

Ferroelectric crystals always exhibit the piezoelectric property, which is the mechanical response of a crystal to an electric field or electrical response to a mechanical stress. With no external field, the centres of the positive and the negative charges are separated by a distance d. If a compressive stress is applied to the crystal, d decrease and a potential difference V develops between the two ends of the crystal. V will be of the opposite sign, if a tensile stress is applied. This effect is known as piezoelectric effect. Conversely, if an external voltage is applied to a ferroelectric crystal, the separation distance d increases (or decrease), thereby elastically straining the crystal. This is called the inverse piezoelectric effect. Such crystals are very much used for frequency controls in radio transmitters and in electronic clocks.