engleski

The Curious Case of Polaronic Transport in Vanadium Phosphate Glasses Containing Transition Metal Oxides

It is well known that oxide glasses containing transition metal oxides (TMO) such as V2O5, MoO3, WO3, and Fe2O3 exhibit electronic conductivity with a small polaron hopping mechanism. In these glasses, the polaronic conductivity is a result of electron transfer between TM ions in different oxidation states, and it is dependent on the amount of TMO, the fraction of reduced TM ions, and the average distance between TM ions. This study examines the effect of adding a second TMO on the polaronic conductivity of vanadium phosphate glasses. In that regard, 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 melt quenching. The structure and electrical properties of the prepared glasses were studied by Raman and impedance spectroscopies, whereas Differential Thermal Analysis was used to investigate their 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. As V2O5 is gradually replaced by TMO, each glass series exhibits different electrical behaviour. 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 in the mixed MoO3-V2O5-P2O5 and WO3-V2O5-P2O5 compositions is a direct consequence of a reduction in the concentration of vanadium ions since molybdenum and tungsten ions do not participate in the polaron transport in these glasses, and therefore cannot compensate for the decrease in V2O5. The high conductivity of binary 60WO3-40P2O5 glass is, however, attributed to the structural peculiarities of this glass system, namely the tendency of tungsten units to form clusters within which polarons are transported. In contrast, increased conductivity of glasses containing >27 mol% of Fe2O3 suggests that iron ions are actively involved in conduction. 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+).

anadium phosphate glass ; polaronic transport ; impedance spectroscopy

nije evidentirano