14.4.5 Pyroelectricity
Pyroelectricity is strongly related to ferroelectricity in the way that in both cases, one needs a material
that consists of ions. At high temperatures, the crystal has a high symmetry crystal structure like a
cubic structure and the centre of the positive charges in the unit cell is the same as for the negative
charges and no spontaneous polarization can be observed. If this material is cooled down however,
it will undergo a structural phase transition and end up in a lower symmetry crystal structure like a
tetragonal structure, where the centres of the positive and the negative charges don’t coincide anymore,
which leads to a polarization. Generally speaking one can state, that all ferroelectrics go through a
phase transition when they are cooled down, which leads to a change in the polarization.
Pyroelectric materials are used as sensors to detect heat for example and two of the most common
pyroelectrics areZnOandLaTaO 3
Fig. 135 shows the specific heat ofPbTiO 3 , a material that can be counted as a pyroelectric as well as
a ferroelectric. The characteristic peak marks the point where the phase transition takes place. Above
the transition temperature, there is no spontaneous polarization and because of that, the pyroelectric
constant ∂P∂T, which is plotted as a function of the temperature in fig. 136(a) vanishes. Another in-
teresting quantity to look at is the dielectric constant, which is proportional toT−^1 TC, whereTCis
the critical temperature at which the phase transition occurs. This constant doesn’t vanish for high
temperatures, but it has also got a peak at the phase transition temperature. The dependence of
on the temperature can be seen even better if one plots^1 as a function ofT, in which case it’s just a
linear function. (fig. 136(b))
Figure 135: Specific heat ofPbTiO 314.4.6 Antiferroelectricity
Some structures, likePbZrO 3 , also have a cubic structure at high temperatures and undergo a struc-
tural phase transition like pyroelectrics. But in contrast to pyroelectric materials antiferroelectrics
don’t have a macroscopic polarization, because the dipols in the unit cells arrange themselves antipar-
allel, hence the net polarization of the whole crystal vanishes.
Fig. 137 is a graphical summary of what was said in the last sections. As one can see, pyroelectrics
and ferroelectrics behave the same at temperatures above and below the critical temperature. If an
electric field atT < TCis applied however, the polarizations in the pyroelectric unit cells won’t change,
whereas the polarization in the ferroelectric unit cells amplifies the outer field by aligning parallel.