Figure 1
APS/Joan Tycko

Figure 1: Ferroelectric materials, such as PbTiO3, acquire an electric polarization because the crystal lowers its energy (blue curve) when positively charged ions (green and blue) displace relative to negatively charged ions (red). (Displacement of Pb ions not shown, for clarity.) In most materials this energy gain is overwhelmed by the energy cost to the crystal (yellow curve) in the presence of the depolarization field—the internal electric field that comes from the displaced ions.