Various dopants were added to BaZrO[3] and the conductivities, the proton concentrations, the site occupancy of the dopants and the change in lattice volume as a result of chemical expansion were investigated. Lanthanide group dopants occupied both the Ba and Zr sites, but the amount of these dopants in the Ba site was too limited to significantly influence the conductivity. The samples doped with Yb, Tm, Er, Y and Ho showed both high proton concentrations and high conductivities, together with a relatively large lattice expansion as a result of hydration. We therefore suggest that, in most instances, the proton concentration, proton conductivity and lattice change as a result of chemical expansion were all correlated in proton conductive acceptor-doped BaZrO[3]. However, Sc-doped BaZrO[3] seemed to be different. Its proton concentration was high, but the conductivity and lattice change as a result of chemical expansion were relatively small. This indicates that the conductivity was strongly related to the lattice expansion resulting from hydration rather than simply the proton concentration.
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