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.