Effect of partial substitution of In by Zr, Ti and Hf on protonic conductivity of BaInO2.5

The effect of partial substitution of In by Zr, Ti and Hf in BaInO2.5 has been studied in order to improve its protonic conductivity. The isotope effect of H and D on the protonic conductivity has also been studied, which indicates that hydrogen diffusion is a rate-limiting process for the protonic conduction. Although the contents of substituting elements do not correlate well with the variation of the protonic conductivity, a 'free volume', which is usually introduced to interpret changes of the conductivity of oxide ion conductors, can do well. This suggests the free volume could be used as a figure of merit to improve the protonic conductivity in BaInO2.5 based oxides.