This effect is often observed in insulators with different phases in them and these different phases have
different dielectric constants. If one phase has a much higher resistivity than the others, the whole
material will be an insulator. Between these phases there are other phases, which conduct a little bit.
So the whole thing looks like a chain of resistors and capacitors and this has a specific AC response.
Total polarizability:
In fig. 129 the parts of the different polarizabilities are shown in dependence of the frequency. First it
has to be said, that the part of the space charge polarizability is not from big technical interest and
so it is not plotted, but it would be at low frequencies.
The important thing is, that if the frequency is increased, there is a point where one can observe a
behaviour that can also be seen when the charges in an atom oscillate naturally. The oscillations are
so fast, that they get out of phase and the polarizability gets negative. This is a typical response of a
mass-spring system. When the applied force oscillates at low frequencies, the masses move with the
force and at high frequencies the masses move out of phase to the applied force.
Figure 129: Diagram of the different polarizability parts of the total polarizability14.4 Structural phase transitions
14.4.1 Introduction
This chapter is about structural phase transitions and the consequences to the dielectric constant. A
structural phase transition is a transition from one crystal structure to another, for example from fcc to
bcc. The stability of the crystal structure of a certain material is often dependent to the temperature.
So it could happen, that the structure changes, if the temperature changes.