D. C. Conduction and Low Frequency Dielectric Absorption of Electronically Conducting Oxide Glasses in Low Temperature Region

Abstract

According to the polaron theory, basic process of electron (or hole) transfer in polar solids transforms from hopping motion into tunnel effect with lowering temperature. In order to investgate this transformation of the electron transfer process in various electronically conducting oxide glasses containing transition metal oxide, the d. c. conductivity and the dielectric properties in low temperature region were studied as a function of temperature and frequency. Measurements were performed over the frequency range from d. c. to 106 Hz at various constant temperatures in the range from 20 to -180°C. Experimental results were discussed in terms of the theory of dielectrics and the polaron concept.(1) Irrespective of the kind of the transition metal oxides in the glasses, a critical temperature Tc is found in the region of temperature -70° to -80°C on the plot of logarithm of the d. c. conductivity against the reciprocal temperature: the activation energy for conduction at tempertures below Tc is lower by 30% than that above Tc. And so there appear to be some changes in conduction process at temperatures below Tc.(2) Extend over wide temperature range, the nature of low frequency dielectric absorption has the features in common with the previously reported features of dielectric relaxation in various ionically and electronically conducting oxide glasses. That is; the activation energy for dielectric relaxation is nearly identical with that for d. c. conduction.And there is a correlation between the d. c. conduction and dielectric absorption as follows;σ≅ε0⋅Δε⋅2πfmaxwhere σ is the d. c. conductivity, ε0 the dielectric constant for vacuum, Δε the magnitude of dielectric absorption and fmax the frequency at loss maximum. The measure ε″max/Δε for representation of the spectral spread of the absorption is approximately 0.2, and is independent of temperature as was observed in many glasses. Moreover, this nature of the dielectric relaxation does not change at all even in temperature region below Tc.(3) If the coexistance of hopping motion and tunnel effect contributes to the decrease of the activation energy for d. c. conduction in temperature region below Tc, the nature of the dielectric relaxation of the glasses, which may be closely related to conduction process, must change remarkably. However, any drastic change in the relaxation behavior is not observed at temperatures below Tc. Therefore, it seems likely that hopping motion yet predominates even in temperature region below Tc.