engleski

Polaronic transport in vanadium phosphate glasses containing transition metal oxides

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, three series of ternary xTMO–(60–x)V2O5–40P2O5 glasses with a wide range of compositions, x=10-60 mol% for TMO=WO3 and MoO3 and x=10-45 mol% for TMO=Fe2O3, 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 SQUID magnetometer. Each glass series shows different electrical behaviour upon gradual substitution of V2O5 by TMO. While with the addition of MoO3, conductivity continuously decreases over three orders of magnitude, it shows a deep minimum at ~53 mol% of WO3 and ~27 mol% of Fe2O3. The observed decrease in conductivity over the entire range of mixed MoO3-V2O5-P2O5 and WO3-V2O5-P2O5 compositions is directly related to a decrease in the concentration of vanadium ions which have a determining role in the electrical processes. It seems that molybdenum and tungsten ions do not participate in the polaron transport in these glasses and hence cannot compensate for the decrease in V2O5 content. However, the high conductivity of binary 40WO3-60P2O5 glass is related to the structural peculiarities of this glass system manifested by the tendency of tungsten units to form clusters within which the transport of polarons is facilitated. On the other hand, a substantial increase in conductivity above 27 mol% of Fe2O3 suggests that iron ions actively contribute to the conduction process. Since the binary Fe2O3-P2O5 glasses exhibit significantly lower polaronic conductivity (<10-9 (Ω cm)-1) than our mixed Fe2O3-V2O5-P2O5 compositions, it can be inferred that a high conductivity of the latter glasses originates from the polaron hopping between various transition metal ions (V4+-V5+, Fe2+-Fe3+, Fe2+-V5+ and V4+-Fe3+).

vanadium phosphate glass ; transition metal oxides ; electronic conductivity ; structural properties

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