A versatile role of WO3 and MoO3 in electrical transport in phosphate glasses

Abstract

Phosphate glasses containing transition metal oxides such as MoO3 and WO3 are well-known for their semiconducting nature with polaronic conduction mechanism. These glasses can also accommodate a relatively high amount of alkali and silver oxides which give rise to ionic conductivity. Such a large compositional and preparation variability enables tuning of the types and mechanisms of the electrical conduction and makes these materials attractive for application in modern electrochemical devices. In this contribution, we discuss various factors that influence electrical transport in these glasses, from simple binary WO3/MoO3-P2O5 systems to complex ones containing variable amounts of alkali/silver oxides. Interestingly, WO3 and MoO3 can have very different roles in the electrical conduction in these materials. While MoO3 contributes weakly via polaronic transport in phosphate glasses, it strongly increases the mobility of alkali/silver ions by forming the mixed phosphate-molybdate network. On the other hand, the role of WO3 depends on the glass composition; it can contribute either directly, by introducing significant polaronic conductivity, or indirectly, by facilitating the transport of alkali/silver ions. The former behaviour is related to the formation of clusters of tungsten units in the glass network which enable fast transport of polarons whereas the latter one originates from the facilitating effect of mixed phosphate-tungstate units on the dynamics of alkali ions.