The first measurements are reported for the frequency-dependent ac conductivity for the iron bismuthate glassy semiconductors in the frequency range 102-105 Hz and in the temperature range 80-450 K. The experimental data have been analyzed with reference to various theoretical models based on quantum-mechanical tunneling through the barrier and classical hopping over the barrier. The analysis shows that the correlated-barrier-hopping model is the most appropriate for the material under consideration. This model predicts quantitatively the temperature dependence of both the ac conductivity and its frequency exponent. However, other models, such as the quantum-mechanical tunneling model, are consistent with the low-temperature ac conductivity, but completely fail to interpret the observed temperature dependence of the frequency exponent. Similarly, the overlapping-large-polaron tunneling model can explain the temperature dependence of the frequency exponent at low temperature, although this model predicts the temperature dependence of the ac conductivity to be much higher than what the experimental data show
