Ferroelectricity

Ferroelectricity is a property found in certain solid-state materials in which a spontaneous electric polarization exists and can be reversed by applying an external electric field. This switchable polarization distinguishes ferroelectrics from ordinary dielectrics and places them in close analogy to ferromagnets, which exhibit spontaneous magnetization. Ferroelectricity was first identified in 1920 in Rochelle salt by the American physicist Joseph Valasek. Although the prefix ferro- implies the presence of iron, most ferroelectric materials do not contain iron. Materials that combine ferroelectric and ferromagnetic ordering are known as multiferroics.
Ferroelectrics are inherently piezoelectric and pyroelectric because their lack of inversion symmetry gives rise to internal dipole moments that respond to mechanical stress and temperature changes. Their ability to maintain, modify and reverse polarization makes them essential in a wide range of modern technologies, including capacitors, sensors and non-volatile memories.

Polarization Behaviour

When a typical dielectric is subjected to an external electric field, the induced polarization increases linearly with the field strength, and the electric permittivity remains constant. However, many materials exhibit nonlinear polarization and are referred to as paraelectric. Ferroelectric materials go further: they retain a spontaneous polarization even in the absence of an applied field. This non-zero polarization arises because the charge distribution within each unit cell of the crystal is asymmetric.
A defining feature of ferroelectrics is that their polarization is reversible. Applying a sufficiently strong opposing electric field flips the direction of the spontaneous polarization, producing a characteristic hysteresis loop when polarization is plotted against electric field. The hysteresis reflects the dependence of polarization not only on the present field but also on the field history.
Ferroelectricity typically occurs only below a critical temperature known as the Curie temperature (T₍C₎). Above T₍C₎, the material becomes paraelectric and usually adopts a centrosymmetric crystal structure, eliminating both spontaneous polarization and pyroelectric effects.

Applications of Ferroelectric Materials

The high and tunable permittivity of ferroelectrics allows them to be used in capacitors with adjustable capacitance. Ferroelectric capacitors consist of a thin film of ferroelectric material sandwiched between two electrodes. Because their permittivity increases greatly near the phase transition temperature, these capacitors can achieve high capacitance in compact form.
The hysteresis and memory effect in ferroelectrics are exploited in ferroelectric random-access memory (FeRAM) used in computers and contactless identification cards. Thin films are essential in such devices because they permit polarization switching under moderate voltages, though care must be taken to manage interfaces, electrode quality and structural defects.
Due to their simultaneous ferroelectric, piezoelectric and pyroelectric properties, these materials lend themselves to various sensing technologies. They are employed in:

• medical ultrasound transducers,
• infrared detectors capable of measuring extremely small temperature variations,
• fire and vibration sensors,
• sonar systems,
• fuel injection components in diesel engines.

Emerging technologies include ferroelectric tunnel junctions, which utilise nanometre-thick ferroelectric layers between metal electrodes to create switchable electronic behaviour, and multiferroic devices, which attempt to couple electric and magnetic ordering in novel ways.
Ferroelectric surfaces also influence catalytic reactions. Polarization-dependent charge at the surface can modify adsorption energies, offering a means of surpassing the limitations of the Sabatier principle by enabling surfaces to switch between strong and weak adsorption states. The same polarization effects can enhance photocatalysis by promoting the separation of photogenerated charge carriers. Pyroelectric and piezoelectric contributions from temperature changes or mechanical vibrations may further drive chemical reactions. Additionally, ferroelectric materials can store optical information via photoferroelectric imaging, producing non-volatile and erasable images.

Fundamental Material Behaviour

The dipoles within a ferroelectric crystal are strongly coupled to lattice distortions. Any external influence that alters the lattice, such as stress or temperature, changes the spontaneous polarization and hence the surface charge. This can induce measurable currents even without an applied voltage. The presence of an asymmetric unit cell underpins the pyroelectric and piezoelectric effects, with the former referring to polarization change with temperature and the latter to polarization change with mechanical stress.
Of the 230 crystallographic space groups, 32 crystal classes exist. Twenty-one of these are non-centrosymmetric, and twenty are piezoelectric. Of these, ten exhibit temperature-dependent spontaneous polarization, making them pyroelectric. Ferroelectrics form a subset of the pyroelectric materials—those in which the spontaneous polarization is reversible.
Ferroelectric phase transitions can be broadly categorised into displacive and order–disorder types, although many materials show characteristics of both. In displacive systems such as barium titanate (BaTiO₃), the transition arises from a “polarization catastrophe”, in which a small displacement of ions leads to increased electrostatic forces that overcome elastic forces, creating a permanent dipole. In BaTiO₃, this involves the titanium ion shifting within an oxygen octahedron.
In lead titanate (PbTiO₃), although the crystal structure resembles that of BaTiO₃, interactions between lead and oxygen atoms complicate the ferroelectric behaviour. In order–disorder systems, each unit cell contains a dipole at all temperatures, but above T₍C₎ the dipoles are randomly oriented. On cooling, they align to form ferroelectric domains.
A technologically important material is lead zirconate titanate (PZT), a solid solution between ferroelectric lead titanate and antiferroelectric lead zirconate. Different compositions are selected depending on application: those richer in lead titanate are suited to memory functions, while compositions near the morphotropic phase boundary exhibit enhanced piezoelectric properties desirable for actuators and sensors.