Different types of defects are there in a crystal,so based on the defect dielectric constant will change.
The dielectric constant is about Molecule but frequency is about bond, but the studies of the frequency and temperature dependence of the dielectric properties unveil useful information about structural changes, defect behavior, and transport phenomena . The dielectric constant and dielectric loss values are decrease with increase in frequency. This is a normal dielectric behaviour and also the basic mechanism of polarization is similar to that of conduction process. This effect can be attributed to the effect of charge distribution by mean carrier hopping on defects. At low frequency, the charge on the defects can be rapidly redistributed so that defects closer to the positive side of the applied field become negatively charged, while defects closer to the negative side of the applied field become positively charged. This leads to a screening of the field and overall reduction in the electric field. Since capacitance is inversely proportional to the field, a reduction in the field for a given voltage results in an increased capacitance as the frequency is lowered. At high frequency, the defects no longer have enough time to rearrange in response to the applied voltage, hence the capacitance will decreasesso, the value of dielectric constant also depends on defect level of material.
Dear Raja
Polarization of dielectric material is due to four mechanisms
electronic polarization, ionic polarization, Orientation polarization and Space charge polarization.
Lattice defects lead to space charge polarization. When the external electric field is applied the localized electron movement results in the formation of a small region with net dipole moment. Many such regions contribute to the total space charge polarization. The net polarization of the dielectric is due to the combined effect of all the four polarization mechanisms.
Suggested Reading: Solid State Physics, Book written by S. O. Pillai
Regds
Mahesh
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