Mixed conduction in alkali niobium tungsten phosphate glasses

Glasses of the 45xMO1/2–45(1-x)WO3–25NbO5/2–30PO5/2 (M = Na or K, 0.5 ≤ x ≤ 1.0) system possessing carriers such as electrons, protons, and alkali ions were prepared. Well-oxidized glasses with almost no electrons or protons as carriers exhibited alkali ionic conductivity in the all compositions considered here. The glasses were colored by heat treatment under H2 atmosphere. The treated glasses with lower alkali content showed a remarkable increase in electrical conductivity, whereas those with higher alkali content showed almost no change compared to the well-oxidized state. It was found that there was a strong correlation between the increase in electrical conductivity due to heat treatment under H2 atmosphere and the hydrogen absorption capacity of the glasses. The increase in conductivity of the glasses with lower alkali content was owing to the change of the major carrier from alkali ions to electrons or protons generated by hydrogen absorption. The composition dependence of the electrical conductivity of the treated glasses reached the minimum at the composition where the contributions of ion and electron or proton conductions became almost equivalent. Mixed conduction was realized in the glass exhibiting minimum conductivity.