Electrical transport in mixed ion–polaron glasses
Mixed ionic–polaronic transport has been investigated in two series of glasses containing Na2O and WO3/MoO3, namely xWO3–(30–0·5x)Na2O–(30–0·5x)ZnO–40P2O5 and xMoO3–(30–0·5x)Na2O–(30–0·5x)ZnO–40P2O5, 0≤x≤60 (mol%). The changes in the conduction mechanism with the systematic alternation of the glass composition have been analysed in correlation to the structural modifications and variations of molybdenum and tungsten in different oxidation states. Raman spectra revealed the clustering of WO6 units by the formation of W–O–W bonds in glasses with increasing WO3 content while the co-existence of MoO4 and MoO6 units is evidenced in glasses containing MoO3. In particular, the dominant molybdenum coordination in glasses with highest MoO3 content is four suggesting the absence of clustering of molybdate units. The DC conductivity of glasses with ≤20 mol% of WO3 and ≤30 mol% of MoO3 is almost identical due to the dominance of ionic conductivity. In this compositional region, the introduction of tungstate and molybdate units in the glass structure increases the mobility of sodium ions and compensates the decrease in sodium number density. The significant difference in conductivity is observed for glasses with higher WO3 and MoO3 content. While for glasses containing MoO3 the conductivity remains almost constant up to 50 mol% of MoO3, it constantly increases for almost six orders of magnitude for glasses with up to 60 mol% of WO3. The behaviour of DC conductivity for glasses containing MoO3 suggests the transition from ionic to polaronic conduction mechanisms which transport pathways are independent of each other. On the other hand, significantly higher conductivity of glasses with higher amounts of WO3 originates from a huge polaronic contribution, much larger than in a case of MoO3. Such a high conductivity is a consequence of the clustering of tungstate units which forms continuous W–O–W–O–W bridges which significantly facilitate polaronic transport.
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Document Type: Research Article
Publication date: June 1, 2019