# 28.3: Polarisation

Polarisation may be defined as the total dipole moment per unit volume, i.e.

$P = \frac{\sum \mu}{V}$

Materials are polarised along a unique crystallographic direction, in that certain atoms are displaced along this axis, leading to a dipole moment along it. Depending on the crystal system, there may be few or many possible axes. As it is the most common and easy to see, let us examine a tetragonal system that forms when cooled from the high temperature cubic phase, through the Curie temperature, of e.g. Tc=120°C in BaTiO3.

In this system, the dipole moment can lie in 6 possible directions corresponding to the original cubic axes:

In a crystal, it is likely that dipole moments of the unit cells in one region lie along a different one of the six directions to the dipole moments in another region. Each of these regions is called a domain, and a cross section through a crystal can look like this:

A domain is a homogenous region of a ferroelectric, in which all of the dipole moments in adjacent unit cells have the same orientation. In a newly-grown single crystal, there will be many domains, with individual polarisations such that there is no overall polarisation. They often appear:

The polarisation of individual domains is organised such that +ve heads are held near -ve tails. This leads to a reduction in stray field energy, because there are fewer isolated heads and tails of domains. This is analogous to the strain energy reduction found in dislocation stacking.

Domain boundaries are arranged so that the dipole moments of individual domains meet at either 90°or 180°. In a polycrystal (one with more than one crystallographic grain), the arrangement of domains depends on grain size. If the grains are fine (<< 1 micron), then there is usually found to be one domain per grain. In larger grains there can be more than one domain in each grain.

This is a micrograph showing the domains in a single grain.

This micrograph is reproduced from the DoITPoMS Micrograph Library (entry number: 199).

In this grain, the domains are twinned in such a way as to reduce the overall stray electric field energy. As each domain possesses its own dipole moment, we may switch dipole moments in order to encode information.