Atomic-Scale Design of Grain Boundaries in BaZrO ₃ : Enhancing Proton Transport by Modulating Lattice Disorder and Defect Distribution

The atomistic origin of grain-boundary (GB) resistance in ceramic proton conductors remains unclear, limiting conductivity optimization. This study shows that moderate lattice disorder at the GB can weaken proton localization and deep trapping, thereby improving the connectivity of low-barrier migration pathways. By comparing two contrasting Σ3 GBs as models, we show that the more ordered Σ3(1 1 1)[110] GB forms a proton-trapping core, increasing the migration barrier by up to ∼6 times relative to the bulk. In contrast, the Σ3(1 1 2)[110] GB exhibits a more delocalized migration-barrier landscape with connected low-barrier segments, reducing the optimal-path barrier to about 1.3 times that of the bulk. A conservative estimate further suggests that the proton conductivity normal to the GB in the latter is about 3–5 orders of magnitude higher over 800–1000 K. Electronic-structure analysis links this behavior to O-p band-center shifts and layer-resolved Bader-charge oscillations. Dopant screening shows that Sb, Y, Ir, and In give the largest barrier reductions, whereas Al, Co, and Ga yield much smaller improvements. These results clarify the relationship among GB structure, local electronic response, and proton migration and may inform GB and dopant design in proton-conducting ceramics.