Effect of sulfur addition on the transport properties of semiconducting iron phosphate glasses

Transport properties and redox state of iron in glasses with compositions × S– (40 Fe2O3 – 60 P2O5)(mol%), where x = 0, 2,4,6 and 8 (mass%), were studied. The overall features of the XRD curves confirm the amorphous nature of the present glasses. Sulfur acted as a reducing agent for redox reaction during glass synthesis and affected the conductivity. Mössbauer spectral analyses revealed that the Fe2+ ratio increases with increasing sulfur content. The high temperature above θ/2 (θD Debye temperature) dependence of conductivity could be qualitatively explained by the small polaron hopping model. The physical parameters obtained from the best fits of this model are found reasonable and consistent with the glass compositions. The conduction is confirmed to be due to adiabatic small polaron hopping of electrons between iron ions. The electron–phonon interaction coefficient γp was large (21.42–26.26). The estimated hopping mobility was low, 1.12 × 10−9–24.83 × 10−9 cm2 V−1 s−1 and increased with increasing S(mass%) content. Moreover, the low temperature (below θ/2) conductivity could not be explained either by Mott’s or greaves variable – range hopping model giving rise to unusually large values of the density of states at the Fermi level compared to those of transition metal oxide glasses. The conductivity of the present glasses was primarily determined by hopping carrier mobility.

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► Iron phosphate glasses used as host materials for vitrifying high level nuclear wastes.
► Iron phosphate glasses melted in air are electronically conducting glasses.
► The electron conducting behavior is due to iron ions being in more than valance state.
► Conduction in these glasses takes place by electrons hopping from low to high valance sites.
► The effect of sulfur ion on the redox state of iron ions has rarely been studied.