Effect of partial substitution of in by Zr, Ti and Hf on protonic conductivity of BaInO_2.5

The effect of partial substitution of In by Zr, Ti and Hf in BaInO _2.5 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 BaInO_2.5 based oxides.