Dopant-Host Oxide Interaction and Proton Mobility in Gd:BaCeO3

The local structure of Gd:BaCeO3 at different dopant concentrations (2-20%) was studied by X-ray absorption spectroscopy. The EXAFS analysis shows that the environment of the regular Ba2+ and Ce4+ cations is to a limited extent affected by doping. The local structure of gadolinium shows an expansion of the first coordination shell of oxygens, consistent with the ionic radius of Gd3+, but a contraction of the next neighboring shells of cations. In particular, the Ba2+ second neighbors get closer to the dopant, which can be originated by the effective negative charge sharply localized on the dopant. Comparison between EXAFS data of dry and hydrated compounds confirms this interpretation, showing a strong interaction of Gd3+ with positively charged defects. The environment of gadolinium is compared with the previously investigated local structure of dopants in Y:BaCeO3 and In:BaCeO3. It is observed that yttrium and gadolinium, which induce higher proton conductivity, are characterized by low solubility and strong interaction with positive defects; on the contrary, the lower proton conductivity of In:BaCeO3 coexists with full dopant solubility and insignificant interaction with oxygen vacancies and hydroxyls. A comprehensive interpretation of this behavior is proposed, in terms of a different dopant-host oxide interaction.