engleski

Enhancement of polaronic conductivity in phosphate glasses containing iron (III) oxide by spontaneous crystallization

Oxide glasses containing transition metal oxides (TMO) such as V2O5, MoO3, WO3, and Fe2O3 exhibit electronic conductivity with a small polaron hopping mechanism. The polaronic conductivity in these glasses arises from the transfer of electrons between TM ions in different oxidation states and depends on the amount of TMO, the fraction of reduced TM ions, and the average distance between TM ions. In this study, we investigate the influence of the addition of a second TMO on the polaronic conductivity of vanadium phosphate glasses. For that purpose, a series of ternary xFe2O3–(60–x)V2O5–40P2O5 glasses with a wide range of compositions, x=10-45 mol%, were prepared by the melt quenching method. The structure and electrical properties of the prepared glasses were studied by Raman and impedance spectroscopy, whereas Differential Thermal Analysis investigated thermal properties. The sum fraction of transition metal ions in a lower oxidation state was determined from the temperature dependence of magnetization measured using a SQUID magnetometer. With the addition of Fe2O3, conductivity shows a deep minimum at -27 mol% of Fe2O3 followed by a substantial increase. While a decrease in conductivity is related to the decrease in the number density of vanadium ions which govern the polaronic transport in this compositional range, the increase at higher Fe2O3 fractions originates from two effects: i) active contribution of iron ions to the conduction process via polaron hopping between various transition metal ions (Fe2+−Fe3+, Fe2+−V5+ and V4+ −Fe3+) in the glass network and ii) nanocrystallization of mixed-valence Fe7(PO4)6 which occurs spontaneously during glass preparation. In particular, it was found that a high polaronic conductivity of glass-ceramic containing 45 mol% of Fe2O3 results from the transport of polarons along and through the nanocrystalline Fe7(PO4)6 which contains both Fe2+ and Fe3+ ions.

phosphate glass ; iron(III)oxide ; crystallization ; polaronic conductivity

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