MICROSCOPIC CONSIDERATIONS OF THE CALCULATION OF THE POLARIZATION CONDUCTIVITY IN IONICALLY CONDUCTING GLASSES

We have used a hopping conduction model to calculate the frequency-dependent polarization conductivity sigma'(omega) and we have assumed that the short-range ionic hopping mechanism occurs because of the 'free-volume' redistribution within the glass structure. We have represented the glass structure as a slightly imperfect gas of cells and calculated, using statistical thermodynamic considerations, the influence of the microstructural aggregation of cells on the ionic polarization conductivity. Likewise, we have underlined the fundamental differences between the frequency-dependent polarization conductivity due to 'bound-charge' resonance (local process) and the frequency-independent d.c. conductivity which can be ascribed to 'free-charge' diffusion (non-local process). Therefore our approach differs from those models which do not take into account this difference and treat the a.c. conductivity behaviour as resulting from a single process. The theoretical calculations explain rather well the experimental results obtained on Na2O-3SiO2 and (Ag2S)x(GeS2)1-x ionically conducting glasses and lead to the determination of two parameters-eta and W(m) which are respectively related to the strain throughout the solid and to the potential energy of the charge carriers (0.43 eV for Na+ and 0.26 eV for Ag+).