Relaxation processes in glasses containing transition metal ions

Dielectric relaxation in iron phosphate glasses containing different glass modifiers has been studied in a brood range of frequency and temperature by ac conductivity measurements. The results have been analyzed using the following procedures: (1) fitting the conductivity to power laws in frequency, namely sigma(ac)=A omega(x), and (2) fitting the electrical modulus, M '', to a stretched exponent in Kohlrausch - Williams - Watt (KWW) relation. The parameter, beta, which express the non-exponential nature of the electrical relaxation is substantially different if compare to the value beta obtained from the frequency dependence of conductivity via the coupling model where beta=1-s. It is shown that above discrepancy arises from the fact that M '' peak is dominated by dc conductivity.