Anti Ferroelectricity Advanced Condensed Matter Physics

TOPIC Anti Ferroelectricity CLASS- M. Sc. 4 th sem. UNIT- 05, PAPER CODE- MPHYEC- 1B

BY DR. RASHMI REKHA DEPARTMENT OF PHYSICS C. M. SCIENCE COLLEGE,DARBHANGA Discuss the different aspects of Anti ferroelectricity

A group of solids which gives rise to order dipoles below a characteristic temperature. It does not exhibit spontaneous bulk polarization . Since each dipole is anti - parallel to its neighbouring dipole. This type of materials are known as anti ferroelectric.

As for example:- Lead Zirconite (PbZrO3), Tungsten Trioxide (WO3), ADP: Ammonium Dihydrogen

Phosphate (NH4H2PO4), Sodium Niobate (NaNbO3), etc.

Anti ferroelectricity and TemperatureTemperature: :::----

This section of the topic explains and essential difference between the electrical behaviour of an anti ferroelectric and ferroelectric crystals. The dielectric constant of an anti ferroelectric crystal increases with the decreasing temperature(above the transition temperature).

It can be used in capacitors because of large dielectric constant, as shown below table- 01.

Table - 01 Materials Transition temperature T (K)

PbZrO 3 506

PbHf O3 488

NH 4H2PO 4 148

NH 4H2AsO 4 216

WO 3 1010 The property of anti ferroelectric will vanish above a particular temperature. Thus, it is known as Anti ferroelectric Curie point. The dielectric constant (relative permittivity) less and more than this Curie point is discussed. This is done for both first and second order transition. In the second order transition, dielectric constant is continuous all over the Curie point. In the both cases dielectric constant must not be very high. The materials and their curie temperature are shown as below table- 02.

Table - 02

000 Materials Curie temperature, T CCC in CCC

NaNbO 3 +63

NH 4H2PO 4 -125

PbZrO 3 +233 Properties ofofof anti ferroelectricferroelectric: :::----

The anti ferroelectric property is not piezoelectric. There are no changes in mechanical character of the material by the application of external field . These materials usually have high dielectric constant. The dipole orientation of this material is similar to the chess board pattern which is shown in Fig. No.- 01.

with the application application electricthe field. ofwith . Dipole arrangement in . ferroelectricDipolein arrangement anti material

Fig. No.- Fig. 01 Double Hysteresis LoopLoop: :::----

The hysteresis loop of a perfect anti ferroelectric material has been drawn, which is shown in Fig. No.- 02. The reverse of spontaneous polarization of that materials gives a double hysteresis loops. An external field which is applied gives a low frequency AC field. Fig. No.-Fig. 02 ############# THANK YOU #############