Callister - An introduction - 8th edition

766 • Chapter 18 / Electrical Properties
18.24 FERROELECTRICITY
ferroelectric
The group of dielectric materials called ferroelectrics exhibit spontaneous
polarization—that is, polarization in the absence of an electric field. They are the
dielectric analogue of ferromagnetic materials, which may display permanent
magnetic behavior. There must exist in ferroelectric materials permanent electric
dipoles, the origin of which is explained for barium titanate, one of the most
common ferroelectrics. The spontaneous polarization is a consequence of the positioning
of the Ba 2 ,Ti 4 , and O 2 ions within the unit cell, as represented in
Figure 18.35. The Ba 2 ions are located at the corners of the unit cell, which is
of tetragonal symmetry (a cube that has been elongated slightly in one direction).
The dipole moment results from the relative displacements of the O 2 and
Ti 4 ions from their symmetrical positions as shown in the side view of the unit
cell. The O 2 ions are located near, but slightly below, the centers of each of the
six faces, whereas the Ti 4 ion is displaced upward from the unit cell center. Thus,
a permanent ionic dipole moment is associated with each unit cell (Figure 18.35b).
However, when barium titanate is heated above its ferroelectric Curie temperature
[120C (250F)], the unit cell becomes cubic, and all ions assume symmetric
positions within the cubic unit cell; the material now has a perovskite crystal
structure (Section 12.2), and the ferroelectric behavior ceases.
Spontaneous polarization of this group of materials results as a consequence of
interactions between adjacent permanent dipoles wherein they mutually align, all
in the same direction. For example, with barium titanate, the relative displacements
of O 2 and Ti 4 ions are in the same direction for all the unit cells within some volume
region of the specimen. Other materials display ferroelectricity; these include
Rochelle salt (NaKC 4 H 4 O 6
# 4H 2 O), potassium dihydrogen phosphate (KH 2 PO 4 ),
potassium niobate (KNbO 3 ), and lead zirconate–titanate (Pb[ZrO 3 ,TiO 3 ]). Ferroelectrics
have extremely high dielectric constants at relatively low applied field frequencies;
for example, at room temperature, r for barium titanate may be as high
as 5000. Consequently, capacitors made from these materials can be significantly
smaller than capacitors made from other dielectric materials.
Figure 18.35 A
barium titanate
(BaTiO 3 ) unit cell
(a) in an isometric
projection, and
(b) looking at one
face, which shows
the displacements of
Ti 4 and O 2 ions
from the center of
the face.
0.403 nm
0.006 nm
0.009 nm
0.398 nm
0.398 nm
0.006 nm
Ti 4+ Ba 2+ O 2–
(a)
(b)