Polaronic Mobility in Boron Doped Iron Phosphate Glasses: Influence of Structural Disorder on Summerfield Scaling

The electrical conductivity and dielectric properties of xB(2)O(3)(40-x)Fe2O3-60P(2)O(5) (x = 0-20, mol%) glasses have been investigated by impedance spectroscopy in the frequency range from 0.01 Hz to 1 MHz and over the temperature range from 303 to 523 K. The dc conductivity is strongly controlled by polaron concentration, which depends upon the Fe2O3 content and the average distance between iron ions. The evolution of the complex dielectric permittivity as a function of frequency and temperature was studied. The thermal-activated relaxation mechanism dominated the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron migration between iron ions with the distribution of hopping distances. The deviations from the Summerfield scaling in the ac conductivity and dielectric permittivity spectra have been observed for selected polaronic glasses. These deviations are caused by the local structural disorder and/or relaxation generated in the early stage of B2O3 addition as well as at the initial incorporation of BO4 tetrahedra into phosphate chains.