By pure glasses, one can assume that you are neglecting ionic impurities which are normally present in oxide glasses. In glasses, electronic polarization will always be present similar to crystalline materials. In case of inhomogeneities, space charge polarization may also occur. If structural defects are created as observed in semiconducting chalcogenide glasses, these defect states may behave as dipoles as they are charged defects. You can go through my research gate profile which contains many research papers in dielectric relaxation in chalcogenide glasses. In conclusion, the polarization is case dependent in glasses. Nature of dielectric loss and frequency range where dielectric dispersion will occur depend on the type of glass and defects present in them.

Thank you very much Dr. A. Kumar...Could you suggest me some useful publication in this area?

In amorphous materials, including pure glasses, the electronic polarization is the main fundamental type of polarizations. Additional small contribution may give an ionic polarization as well as space charge polarization may also occur, as Dr. A. Kumar have noticed.

Thank you Prof. Grigas..

In case of crydtalline materials you can read chapter 2 (for insulators and semiconductors ) and chapter 7 (for metals and doped semiconductors) in a book entitled "optical properties of solids" by Mark Fox. Furhtermore, in case of amorphous materials, the electronic polarization and structural defects are the main sources of polarization. In case of oxide glasses, non bridging oygen plays the main role of polarization

Thank you very much Prof. Abdel-Wahab...