Dielectric and Ferroelectric Materials

Dielectric and ferroelectric materials are insulators that polarise in an electric field; ferroelectrics additionally possess a spontaneous polarisation that can be switched, giving them high permittivity, piezoelectric coupling, and memory behaviour.

Definition

A dielectric is an insulating material that develops an electric polarisation in an applied field; a ferroelectric is a dielectric with a spontaneous polarisation, arising from a non-centrosymmetric structure, that can be reversed by an external field.

Scope

This topic covers the chemistry of polarisable insulators: the mechanisms of dielectric polarisation and permittivity, the symmetry requirements for piezoelectricity, and the spontaneous, switchable polarisation of ferroelectrics exemplified by perovskite titanates such as barium titanate. It treats the Curie transition between ferroelectric and paraelectric states, domains and hysteresis, and how composition tunes these oxides for capacitors, actuators, sensors, and memories.

Core questions

• What mechanisms give a dielectric its permittivity?
• What crystal symmetry is required for piezoelectricity and ferroelectricity?
• How does spontaneous polarisation arise and switch in a ferroelectric?
• How does the Curie transition link ferroelectric and paraelectric states?

Key concepts

• Dielectric permittivity
• Electronic, ionic, and dipolar polarisation
• Piezoelectricity
• Spontaneous polarisation
• Curie temperature
• Ferroelectric domains and hysteresis

Key theories

Polarisation mechanisms and permittivity

An applied field displaces charge in a dielectric through electronic, ionic, and dipolar polarisation, storing energy and raising the effective capacitance; the magnitude and frequency response of permittivity reflect which mechanisms operate.

Ferroelectricity from a polar distortion

Below the Curie temperature, perovskites such as barium titanate adopt a non-centrosymmetric structure with a spontaneous, switchable polarisation; the associated soft-mode distortion gives very high permittivity and the piezoelectric coupling exploited in devices.

Mechanisms

In ferroelectric perovskites a small off-centre displacement of the cation creates a dipole; below the Curie temperature these dipoles align into domains, giving spontaneous polarisation, and an applied field reorients the domains, producing the hysteresis loop and the strong piezoelectric strain used in devices.

Clinical relevance

Dielectric and ferroelectric materials underpin multilayer ceramic capacitors, piezoelectric sensors, actuators, and ultrasound transducers, ferroelectric and high-permittivity gate dielectrics in microelectronics, and non-volatile ferroelectric memories, with composition chosen to set permittivity, Curie temperature, and coupling.

History

Piezoelectricity was discovered by the Curie brothers in 1880, and ferroelectricity was first recognised in Rochelle salt in the 1920s. The wartime discovery of ferroelectricity in barium titanate, and Megaw's structural studies of its perovskite distortion, established the oxide ferroelectrics that now dominate electroceramic capacitors and piezoelectric devices.

Key figures

• Jacques Curie
• Pierre Curie
• Helen Megaw

Seminal works

• moulson2003
• callister2018

Frequently asked questions

What is the difference between a dielectric and a ferroelectric?

All ferroelectrics are dielectrics, but an ordinary dielectric only polarises while a field is applied and returns to zero polarisation when removed. A ferroelectric has a spontaneous polarisation even with no field, and that polarisation can be switched between stable orientations, giving memory and hysteresis.

Why does a ferroelectric lose its special properties when heated?

Above its Curie temperature a ferroelectric transforms to a higher-symmetry, centrosymmetric paraelectric structure in which the polar distortion vanishes. With no spontaneous polarisation, ferroelectric and piezoelectric behaviour disappear until the material is cooled back below the Curie point.