Ferroelectricity

An electric dipole has separated positive and negative charge. In a ferroelectric crystal, the crystal structure allows a net spontaneous polarization even when no external electric field is applied. Neighboring regions can orient their polarization in different directions; these regions are called domains. The macroscopic polarization of a sample depends on the arrangement of those domains.

An applied electric field can favor domains aligned with it and switch the overall polarization. This ability to reverse spontaneous polarization distinguishes ferroelectrics from ordinary dielectrics, which may become polarized in a field but do not have switchable spontaneous polarization. Barium titanate and Rochelle salt are familiar examples, although ferroelectric materials encompass a wider range of crystal structures and compositions.

When the applied electric field is cycled, polarization does not generally follow the same path as the field increases and decreases. The resulting polarization–electric-field curve is a hysteresis loop, reflecting the lag involved in domain reorientation and switching. Its shape and characteristic quantities depend on the material, sample and measurement conditions.

Ferroelectric order exists over a material-specific temperature range. Above its Curie temperature, thermal motion disrupts the ordered polar state, and the material loses its spontaneous ferroelectric polarization. In barium titanate, for example, a structural change below its Curie point shifts the positions of ions in the crystal and produces polarization; the transition temperature is about 120 °C, though temperatures and transitions differ among materials.

Ferroelectricity is related to, but not identical with, piezoelectricity and pyroelectricity. Piezoelectric materials develop electrical polarization in response to mechanical stress, and can conversely deform under an applied electric field. Pyroelectric materials show a change in polarization with temperature. Ferroelectric materials are also pyroelectric and piezoelectric in their polar phase, but the defining feature of ferroelectricity is that an electric field can reverse the spontaneous polarization.

Ferroelectric ceramics can serve as high-permittivity dielectrics in capacitors. Their coupling between electrical and mechanical behavior is useful in devices such as sensors and actuators. Ferroelectric random-access memory uses switchable polarization to represent stored states, and thin-film ferroelectrics are studied for microsystems and high-frequency electronics.

Research continues to identify materials with combinations of properties suited to particular applications, including lead-free piezoelectric behavior and integration with silicon-based electronics. Computational screening can help find candidates, but predicted switchability is not equivalent to demonstrating a useful device: material synthesis, stability, switching behavior and device-level performance all require assessment.