Ion dynamics in glass-forming systems: II. Conductivity spectra above the glass transformation temperature

The conductivity spectra of three supercooled glass-forming molten salts, LiCl·7H2O, Ca(NO3)2·4H2O and 3KNO3·2Ca(NO3)2, have been taken at various temperatures above T G. The data cover a wide frequency range from a few MHz to the far infrared. The d.c. conductivities exhibit Vogel-Fulcher-Tammann temperature dependences, but with increasing frequency a change to Arrhenius is observed. The crossover occurs along with the onset of the power-law frequency dependence of the conductivity. Increasing the frequency further up to the far infrared, we observe a second crossover, now into a v 2 frequency dependence due to the excitation of vibrational motion. The vibrational contributions can be subtracted out of the spectra. This has been carefully done for 3KNO3·2Ca(NO3)2. The resulting non-vibrational conductivity attains a thermally activated ‘high-frequency’ plateau at about 1 THz, as in a solid electrolyte. On a short time scale, the dynamics of glass-forming fragile melts is, indeed, found to be very similar to that of glass.