Proton and O vacancy hopping, dopant clustering, and octahedral tilt transitions in exsolved BaCe0.7Zr0.1Y0.15Ni0.05O3−δ

Exsolution of metallic nanoparticles from proton-conducting oxides enhances catalytic performance but adds complexity to ionic transport. This study investigates the microscopic processes in BaCe0.7Zr0.1Y0.15Ni0.05O3−δ (BCZYNi) using dielectric and anelastic spectroscopy under wet and dry conditions, before and after Ni exsolution, with BaZrO3 and BaCe1−xYxO3−δ as reference systems. Elastic anomalies in the complex Young’s modulus identify structural transitions driven by oxygen-octahedra tilting, while anelastic relaxation peaks reveal the thermally activated hopping of protons and oxygen vacancies (VO). In BCZYNi, incomplete hydration (57% of the theoretical limit) is confirmed by the persistence of VO under wet conditions. Dielectric spectra exhibit near-Debye relaxations with activation energies of 0.51 eV (hydrated) and 0.84 eV (outgassed). The magnitude and timescales of these relaxations suggest long-range transport modes where H and VO span several lattice spacings, rather than localized dipolar reorientation. The findings demonstrate that dopant clustering is determinant in controlling transport and hydration: while nearestneighbor dopants trap VO and suppress hydration, extended dopant aggregates promote locally fast migration of H and VO without the need for detrapping. Furthermore, a previously unreported dielectric transition near 260 K and the observed structural transition temperatures suggest a role of dopant distribution beyond tolerance-factor predictions.