AC CONDUCTION IN SOL-GEL-DERIVED GLASSES IN THE SIO2-AS2O3 SYSTEM

ac-resistivity measurements on sol-gel-derived glasses in the system xAs2O3.(1-x)SiO2 with 0.05 < x < 0.16 have been carried out over the frequency range 2-100 kHz in the temperature range 80-400 K. The ac resistivity for all the glasses shows a sharp minimum at around 317 K. This is ascribed to a change in chemical equilibrium of the ratio [As5+]/[As3+] as a function of temperature. In the temperature range 190-300 K the ac resistivity is shown to arise due to a correlated-barrier-hopping mechanism involving bipolarons. A small fraction of total arsenic sites is found to participate in the hopping conduction. The ac resistivity at temperatures below 190 K shows a frequency exponent s, the variation of which can be qualitatively explained on the basis of the overlapping-large-polaron model.