Glasses in the system (Na2B4O7)75(Fe2O3)25-x(Bi2O3)x with x=0,5,15 and
25 mol % were prepared. A.c. Conductivity (σa.c) of the prepared samples has
been measured in the frequency range (0.5-20 kHz). The measurements were
carried out in the temperature range (290-420K). The obtained results showed
that, σa.c. obey the relation σa.c(ω)=Aωs, and the exponent (s) was found to
decrease with increasing temperature. The values of s of all the investigated
samples lie between 0.3 and 0.7. The analysis of the results reveals that, the
a.c. conductivity of the investigated samples follow the correlated barrier
hopping (C.B.H) model. The dependence of the dielectric constant (έ) and
dielectric loss (tanδ) on both temperature and frequency are also discussed in
the framework of the current theories. The effect of