Electrical conductivity of semiconducting barium aluminophosphate glasses containing single and double redox pairs

Abstract

Α steep increase of conductivity with increasing vanadium oxide concentration up to values 10^-1 S/m at 150 °C is observed in barium-rich glasses of the pseudo-binary systems TM(Transition Metal)-oxide-Ba(PO3)2-Al(PO3)3 with TM-transition metals vanadium, iron, manganese which is similar to that in pure vanadium phosphate glasses. For high concentrations (25 to 80 mol% vanadium oxide) an adiabatic hopping process between V4+ and V5+ ions is found. No hopping process takes place between V4+ and V3+ ions. The system with iron oxide content also shows a steep increase of conductivity. Maximum values around 10^-4 S/m are reached at 150 °C, because only 30 mol% iron oxide are soluble without crystallization. The hopping process between Fe2+ and Fe3+ ions is a non-adiabatic one. No or negligibly small hopping processes seem to exist in the manganese oxide-containing system. In systems with TM-oxides incorporated in pairs the probability of hopping processes significantly depends on the oxidation potential or on the electrochemical series of the redox pairs, respectively, because the electronic charge carrier concentration depends on the mutual shift of the redox equilibria. Those hopping processes which are not conform to this shift have a very low probability. Therefore, increasing or decreasing conductivity in those mixed redox pairs containing systems depends primarily on the electrochemical series and on the initial redox situation of that redox pair and its redox shift, which dominates the electrical conductivity. In the case of Fe2+ ions it was found that an increase in conductivity additionally takes place by a coordination change from 4- to 6-fold in the presence of manganese, favouring the hopping process between 6-fold coordinated Fe2+ and 6-fold coordinated Fe3+ ions.