The glass containing transition metal oxides such as Fe₂O₃, V₂O₅, WO₃, and MoO₃ exhibits electronic conduction due to electron hopping between transition metal ions. When alkali oxides such as Li₂O, Na₂O, and K₂O are added to such a glass, its electrical conductivity changes from electronic, mixed ionic and electronic to ionic with increasing alkali oxide content [1]. Because the mixed ionic and electronic conductor has a potential use as an electrode in Li⁺ and Na⁺ ion batteries, especially all solid-state batteries [2], the glass containing both transition metal and alkali oxides has recently attracted attention. Protons are monovalent cations that dissolve in glass in large quantities, as do alkali ions. Although the glass is expected to be used as electrodes in fuel cells when the glass is the mixed proton and electron conductor, there have been few studies on the mixed protonic and electronic conduction in glass [3] because it is difficult to obtain glass with high concentration of protons.

  In this study, 30HO_1/2−6VO_5/2−14VO₂−50PO_5/2 glass was prepared with an expectation that it would be a mixed protonic and electronic conductor by electrochemical substitution of sodium ions with protons (APS) [4] of the 30NaO_1/2−10VO_5/2−10VO₂−50PO_5/2 glass. While the 30NaO_1/2−10VO_5/2−10VO₂−50PO_5/2 glass was a mixed ionic and electronic conductor, the 30HO_1/2−6VO_5/2−14VO₂−50PO_5/2 glass was a pure proton conductor contrary to the expectation. The suppression of the electronic conduction in the 30HO_1/2−6VO_5/2−14VO₂−50PO_5/2 glass was investigated on the basis of Mott’s theoretical expression of the small polaron hopping conduction in semiconducting glasses. From the evaluation of the parameters in the theoretical expression and the analysis of the glass structure, the suppression was attributed to the increase in the electron wavefunction decay constant induced by the structural change in the vanadophosphate flamework. Specifically, the vanadium ions exist mainly as the VO₄ in the 30NaO_1/2−10VO_5/2−10VO₂−50PO_5/2 glass and VO₆ in the 30HO_1/2−6VO_5/2−14VO₂−50PO_5/2 glass. And the electron wavefunction decay constant for the electron on the vanadium atoms is larger in the VO₆ than in the VO₄. Therefore, in the 30HO_1/2−6VO_5/2−14VO₂−50PO_5/2 glass, where the VO₆ with fast electron wavefunction decay is present as the main form of vanadium oxide, the electron wavefunctions between adjacent vanadium ions do not overlap, resulting in a very small electronic conductivity, even though the distance between vanadium ions and the fraction of V⁴⁺ ions in the total V atoms are almost the same as in the 30NaO_1/2−10VO_5/2−10VO₂−50PO_5/2 glass. The proton conductivity of the 30HO_1/2−6VO_5/2−14VO₂−50PO_5/2 glass was as low as 1×10⁻⁵ Scm⁻¹ at 300 °C compared to conventional proton-conducting phosphate glasses prepared by using APS due to the low proton mobility resulting from the strong proton trapping by the non-bridging oxygen in VO₆.