Dielectric behavior and impedance spectroscopy of bismuth iron phosphate glasses

The electrical and dielectrical properties of Bi2O3-Fe2O3-P2O5 glasses were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 4 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe2O3 content and Fe(II)/Fe-tot ratio. With increasing Fe(II) ion content from 17% to 34% in the bismuth-free 39.4Fe(2)O(3)-59.6P(2)O(5) and 9.8Bi(2)O(3)-31.7Fe(2)O(3)-58.5P(2)O(5), glasses, the dc conductivity increases. On the other hand, the decrease in dc conductivity for the glasses with 18.9 mol% Bi2O3 is attributed to the decrease in Fe2O3 content from 31.7 to 23.5 mol%, which indicates that the conductivity for these glasses depends on Fe2O3 content. The conductivity for these glasses is independent of the Bi2O3 content and arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electronic conduction. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions. The Raman spectra show that the addition of up to 18.9 mol% of Bi2O3 does not produce any changes in the glass structure which consists predominantly of pyrophosphate units. (c) 2005 Elsevier B.V. All rights reserved.