Study of the a.c. conductivity of BaTiO₃-doped V₂O₅-Bi₂O₃ glasses using Long's overlapping large-polaron tunnelling model

Abstract

In the present paper we report the results of a study of the frequency-dependent a.c. conductivity σ_a.c. of the BaTiO₃-doped (5–40 wt%) 90V₂O₅-10 Bi₂O₃ (VB) glassy semiconductor system over a wide frequency range (500-10 000Hz) and wide temperature range (80–450 K). The semiconducting behaviour of these glasses is due to the presence of V⁴⁺ and V⁵⁺ valence states. While the correlated barrier hopping of bipolarons is responsible for the a.c. conductivity in the undoped glasses, Long's overlapping large polaron tunnelling mechanism is found to be valid for all these BaTiO₃-doped VB glass compositions. Such a change in conduction mechanism is not usually observed in transition-metal oxide glasses and it is due to the change in network structure of the VB glass arising from the addition of BaTiO₃. The doped glasses, however, 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-Bi₂O₃ and many other vanadate glasses with the usual glass formers, where an adiabatic hopping conduction mechanism is applicable. The a.c. conductivity follows a power law (namely σ_a.c. α ω ^s with s < 1) for both doped and undoped systems. The model parameters obtained from a least-squares fitting of the experimental a.c. conductivity data also agree quite well with those obtained from other methods.