engleski

Electrical properties of iron phosphate glasses containing HfO2 and CeO2

Glasses based on iron phosphate exhibit electronic conductivity with a small polaron hopping mechanism. The polaron conduction in these glasses includes electron transport from Fe2+ to Fe3+ ions. Consequently, it depends not only on the overall iron oxide content but also on the fractions of iron ions in the two valence states. The fraction of ferrous/ferric ions depends on various factors such as melting conditions (temperature, time, atmosphere), types of raw materials and the presence of other substances in the batch. In this context, it is known that the hafnium and cerium tend to increase drastically the fraction of ferrous ions in iron phosphate melts. In this contribution we discuss the factors that influence DC conductivity as well as features of frequency dependent conductivity of three series of iron phosphate glasses containing HfO2 and CeO2: xHfO2–(40−x)Fe2O3 –60P2O5, 0≤x≤8 mol%, xCeO2–(40−x)Fe2O3–60P2O5, 0≤x≤8 mol%, and xHfO2–(38−x)Fe2O3–2B2O3–60P2O5, 2≤x≤6 mol%. The electrical properties were measured by impedance spectroscopy and fraction of ferrous ions was determined by Mössbauer spectroscopy. While the overall Fe2O3 content varies only slightly within each series (up to 8 mol%), it was found that the fraction of ferrous ions changes from 23% to 58%. The results revealed that DC conductivity of glasses increases linearly with the number density of polarons determined by the Fe2O3 content and the fraction of iron ions in Fe2+ or Fe3+ states. A detailed consideration of the scaling features of frequency-dependent conductivity and permittivity in these systems showed that the Sidebottom scaling procedure yields a super-master curve, implying that while the time–temperature superposition principle is fulfilled, polaronic hopping lengths also change with changing number density of polarons. Further, the spatial extent of local excursions of the polaron calculated from the scaled permittivity spectra is much closer in value to the polaronic radius calculated as per the equation proposed by Bogomolov & Mirilin than to the mean separation between iron ions. Interestingly, the spatial extent for localized polaronic hopping shows a tendency to decrease with increasing charge carrier number density, while no such tendency is observed for the polaron radius calculated from the glass composition and density.

Iron phoshate glasses ; Electrical conductivity ; Impedance spectroscopy

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