Ferroelectricity

Ferromagnetism was already known when ferroelectricity was discovered in 1920 in Rochelle salt by American physicist Joseph Valasek.

In addition to being nonlinear, ferroelectric materials demonstrate a spontaneous nonzero polarization (after entrainment, see figure) even when the applied field E is zero.

The distinguishing feature of ferroelectrics is that the spontaneous polarization can be reversed by a suitably strong applied electric field in the opposite direction; the polarization is therefore dependent not only on the current electric field but also on its history, yielding a hysteresis loop.

They are called ferroelectrics by analogy to ferromagnetic materials, which have spontaneous magnetization and exhibit similar hysteresis loops.

Many ferroelectrics lose their pyroelectric properties above TC completely, because their paraelectric phase has a centrosymmetric crystal structure.

The permittivity of ferroelectrics is not only adjustable but commonly also very high, especially when close to the phase transition temperature.

In these applications thin films of ferroelectric materials are typically used, as this allows the field required to switch the polarization to be achieved with a moderate voltage.

However, when using thin films a great deal of attention needs to be paid to the interfaces, electrodes and sample quality for devices to work reliably.

The combined properties of memory, piezoelectricity, and pyroelectricity make ferroelectric capacitors very useful, e.g. for sensor applications.

Ferroelectric capacitors are used in medical ultrasound machines (the capacitors generate and then listen for the ultrasound ping used to image the internal organs of a body), high quality infrared cameras (the infrared image is projected onto a two dimensional array of ferroelectric capacitors capable of detecting temperature differences as small as millionths of a degree Celsius), fire sensors, sonar, vibration sensors, and even fuel injectors on diesel engines.

Yet another burgeoning application is multiferroics, where researchers are looking for ways to couple magnetic and ferroelectric ordering within a material or heterostructure; there are several recent reviews on this topic.

[14] Sabatier principle states that the surface-adsorbates interaction has to be an optimal amount: not too weak to be inert toward the reactants and not too strong to poison the surface and avoid desorption of the products: a compromise situation.

Photoferroelectric imaging is a technique to record optical information on pieces of ferroelectric material.

The generation of a surface charge in response to the application of an external stress to a material is called piezoelectricity.

In barium titanate, a typical ferroelectric of the displacive type, the transition can be understood in terms of a polarization catastrophe, in which, if an ion is displaced from equilibrium slightly, the force from the local electric fields due to the ions in the crystal increases faster than the elastic-restoring forces.

This leads to an asymmetrical shift in the equilibrium ion positions and hence to a permanent dipole moment.

The ionic displacement in barium titanate concerns the relative position of the titanium ion within the oxygen octahedral cage.

In an order-disorder ferroelectric, there is a dipole moment in each unit cell, but at high temperatures they are pointing in random directions.

In 2010 David Field found that prosaic films of chemicals such as nitrous oxide or propane exhibited ferroelectric properties.

[26] This new class of ferroelectric materials exhibit "spontelectric" properties, and may have wide-ranging applications in device and nano-technology and also influence the electrical nature of dust in the interstellar medium.

Other ferroelectric materials used include triglycine sulfate, polyvinylidene fluoride (PVDF) and lithium tantalate.

[28] It should be possible to produce materials which combine both ferroelectric and metallic properties simultaneously, at room temperature.

[29] According to research published in 2018 in Nature Communications,[30] scientists were able to produce a two-dimensional sheet of material which was both ferroelectric (had a polar crystal structure) and which conducted electricity.

where Px, Py, Pz are the components of the polarization vector in the x, y, z directions respectively, and the coefficients, αi, αij, αijk must be consistent with the crystal symmetry.

To investigate domain formation and other phenomena in ferroelectrics, these equations are often used in the context of a phase field model.

The spontaneous polarization, Ps of a ferroelectric for a cubic to tetragonal phase transition may be obtained by considering the 1D expression of the free energy which is:

We find the derivative of the free energy, and set it equal to zero in order to solve for Ps:

and eliminating solutions which take the square root of a negative number (for either the first or second order phase transitions) gives:

Elimination of this region, and connection of the top and bottom portions of the 'S' curve by vertical lines at the discontinuities gives the hysteresis loop of internal polarization due to an external electric field.

Sliding ferroelectricity is widely found but only in two-dimensional (2D) van der Waals stacked layers.