Ferroelectricity (or polarization catastrophe ) describes the phenomenon that substances with an electric dipole moment change through the application of an external electric field, the direction of the spontaneous polarization. The ferroelectricity was formerly known as Seignette electricity (rare as Seignettesalzelektrizität ), since it was discovered in Rochelle salt ( potassium sodium tartrate ).


Ferroelectricity occurs only in crystals in which the crystalline symmetry allows a polar axis. This results in the displacement of different charged ions in the crystal lattice to the spontaneous polarization. In contrast to piezo- and pyroelectric material, the electrical polarization in the ferroelectric can be reversed by the application of a voltage. Ferroelectric materials are always pyroelectric and thus also piezoelectric.

The prefix " ferrous " refers in the ferroelectrics is not a property of the iron, but the analogy to the ferromagnetism. As in ferromagnetic materials, the magnetization, the polarization disappears at ferroelectrics at high temperatures ( of the ferroelectric Curie temperature ) - The material is then paraelectric. Above this temperature, followed by the relative permittivity analogous to the ferromagnetic susceptibility of the Curie- Weiss law. On cooling of the material, a phase transition takes place on falling below the Curie temperature, which coincides generally with a change in structure (reduction of the crystal symmetry ) and the material is ferroelectric again. In this case, the relative permittivity rises at lower temperatures to significantly at 4 K it is often by a factor of 10 to 20 above the value at room temperature, and can reach values ​​in excess of 106.

Ferroelectric crystals form domains, ie regions with the same polarization direction. From one domain to the polarization direction changes in the range of a few atomic layers, where the polarization vanishes. The ferroelectric domain walls are only a few nanometers wide. In contrast, the orientation of magnetization changes in the ferromagnetism gradually over a range of 10 nm or more. Because of the smaller domain walls may differently oriented domains in the ferroelectric thin films having a higher density than in ferromagnetic thin-films. Therefore, it is hoped that a higher maximum density of information in the development of ferroelectric memory media against ferromagnetic. The polarization by applying an external electric field, as shown in the sketch next to stand, be reversed and follows a hysteresis curve.


Ferroelectrics are used for the production of high precision mechanical actuators ( displacement elements ). Using the inverse piezoelectric effect displacement with a resolution of less than an atomic diameter are possible. They are used for example in atomic force microscopes, scanning tunneling microscopes and other scanning probe microscopes.

Ferroelectrics typically have a high to very high relative permittivity ( ) in the range between 100 and 100,000, which is why they are used as material for ceramic capacitors with high volume capacity. They are increasingly replacing the electrolytic capacitors and are distinguished from these by low equivalent series resistances and inductances (ESR and ESL) from. However, a disadvantage is the strong temperature dependence, the large tolerances and the high dielectric loss factors. This high permittivity makes them interesting in semiconductor technology, where for smaller memory circuits (RAM) high capacity is required in a confined space. The main advantage of the use of so-called FeRAM ( ferroelectric RAM), is that this DRAM their state of charge as compared with currently (2008) mainly used not lose. One speaks in this case of non-volatile memory ( NVRAM). A further area of ​​research is the use of ferroelectrics as a so -called high- k dielectrics. Especially perovskites could substitute as the gate dielectric, the field effect transistors in integrated circuits in the future, the silica. Due to the permanent polarizability they are also suitable as electrets, such B.in sensors and microphones.


The most known ferroelectrics are ion crystals having a perovskite structure, such as:

Furthermore, the following materials are ferroelectric, but only partially in the form of thin films:

  • Strontium bismuth tantalate SrBi2Ta2O9 (SBT)
  • Bismuth titanate Bi4Ti3O12 (BIT, misleading BTO )
  • Bismuth lanthanum titanate Bi4 - xLaxTi3O12 (BLT)
  • Bismuth titanate niobate Bi3TiNbO9 ( BTN)
  • Strontium titanate SrTiO3 ( STO)
  • Barium strontium titanate Ba x Sr 1 - xTiO3 ( BST)
  • Sodium nitrite NaNO2
  • Lithium niobate LiNbO3
  • Potassium sodium tartrate tetrahydrate ( Rochelle salt ) KNaC4H4O6 · 4 H2O

Hexagonal manganites RMnO3 with R = Y, Sc, In, Ho, Er, Tm, Yb, Lu.

It also organic ferroelectrics were found: