Dielectric Constant, Infrared Absorption and Raman Effect of Glassy Fe₂O₃-PbO-SiO₂ System

Abstract

Dielectric constants (DC), infrared (IR) spectra, and Raman spectra of glassy Fe₂O₃-PbO-SiO₂ (Fe₂O₃, 0-20mol%) system were measured. The composition of samples was changed such as xFe₂O₃⋅(1-x) PbO⋅SiO₂ (a-series) or x Fe₂O₃⋅(1-x) (PbO⋅SiO₂) (b-series). The values of DC in a-series increased rapidly from 15.5 (10⁶Hz) of PbO⋅SiO₂ glass up to about 19 of the sample which contained 2mol% of Fe₂O₃. The values of DC in b-series were equal to those in a-series. In the composition range over 2mol% of Fe₂O₃, the value of DC increased slowly up to 15mol% of Fe₂O₃ for a-series and up to 10mol% of Fe₂O₃ for b-series. As Fe₂O₃ content increased moreover, the values of DC for both series increased markedly again. The difference of IR spectra between the samples in both series could not be recognized. IR absorption band shifted from about 950cm^-1 for PbO⋅SiO₂ glass to 920cm^-1 for the sample which contained 20mol% of Fe₂O₃. The shift appeared relatively large at about 2mol% of Fe₂O₃. Raman band of the sample contained Fe₂O₃ under 2mol% showed a similar pattern to that of PbO⋅SiO₂ glass which corresponds to ν₃ mode of SiO₄ vibration. However some split of Raman band occurred when Fe₂O₃ content was larger than 2mol%.
On the basis of above experimental results, the following consideration was given. When Fe³⁺ ion in the glassy system has 6-oxygen neighbors, the Fe³⁺ ion can contribute to the ionic polarization of the system. The induced dipole moment and the change in the ionic field also become effective for the dielectric polarization. Owing these effects, the value of DC increases rapidly with increasing Fe₂O₃ content. IR and Raman bands would not be affected very much since Fe-O interaction is weak when the Fe³⁺ ion has 6-oxygen neighbors. Consequently, in the composition range under 2mol% of Fe₂O₃, it can be suggested that the Fe³⁺ ion in the glassy system has 6-oxygen neighbors. If we assume that the strength of Fe-O bond increases, above effects for the dielectric polarization become relatively few, namely the value of DC would vary gradually with increasing Fe₂O₃ content. Since the symmetry of the SiO₄ vibration is decresed by the increase in the strength of Fe-O bond, the split of its vibrational band would be given rise. As the measured results showed these phenomena, it can be considered that the Fe³⁺ ion having above strong Fe-O interaction also exists when Fe₂O₃ is over 2mol%.