Proton Dynamics in In:BaZrO3: Insights on the Atomic and Electronic Structure from X-ray Absorption Spectroscopy

The local structure of Ba2+, Zr4+, and In3+ in In:BaZrO3 is investigated with EXAFS for samples having 0 to 75% In3+ content. It is found that indium can be inserted in any ratio in the host matrix oxide and that the oxygen coordination shell displays an In-O distance very similar to the Zr-O length. In the Zr-rich compositions, there is a preferred dopant-vacancy association that, however, does not give rise to dopant-proton interaction in the hydrated samples. The tendency of Ba2+ to be attracted toward the dopant site is attributed to the electrostatic interaction with the dopant and to the structural rearrangement around the In3+ site. Third cumulant analysis at high temperatures (up to 673 K) allows to conclude that the anharmonicity of In-O thermal motion is about 1 order of magnitude lower than in other perovskites with higher proton conductivity. It is argued that the lower proton diffusivity displayed by In:BuZrO3 depends on (a) proton trapping at the dopant site due to the formation of a stable O-H center dot center dot center dot O hydrogen bond; (b) reduced anharmonicity of the M-O vibrations; (c) different strength of O-H bonds originated by electronic density rearrangement.