STUDY OF THE AC CONDUCTIVITY OF BATIO3-DOPED V2O5-BI2O3 GLASSES USINGLONGS OVERLAPPING LARGE-POLARON TUNNELING MODEL

In the present paper we report the results of a study of the frequency -dependent a.c. conductivity sigma(a.c.) of the BaTiO3-doped (5-40wt%) 90V(2)O(5)-10Bi(2)O(3) (VB) glassy semiconductor system over a wide f requency range (500-10000Hz) and wide temperature range (80-450 K). Th e semiconducting behaviour of these glasses is due to the presence of V4+ and V5+ valence states. While the correlated barrier hopping of bi polarons is responsible for the a.c. conductivity in the undoped glass es, Long's overlapping large polaron tunnelling mechanism is found to be valid for all these BaTiO3-doped VB glass compositions. Such a chan ge in conduction mechanism is not usually observed in transition-metal oxide glasses and it is due to the change in network. structure of th e VB glass arising from the addition of BaTiO3. The doped glasses, how ever, show phase separation even in the glassy phase in contrast with the pure VB glass. The d.c. conductivity of doped glasses showed a non -adiabatic conduction mechanism in contrast with the cases of V2O5-Bi2 O3 and many other vanadate glasses with the usual glass formers, where an adiabatic hopping conduction mechanism is applicable. The a.c. con ductivity follows a power law (namely sigma(a.c.) (alpha)omega(5) with s < 1) for both doped and undoped systems. The model parameters obtai ned from a least-squares fitting of the experimental a.c. conductivity data also agree quite well with those obtained from other methods.