  Nanostructured proton-conducting ceramics (PCCs) have attracted considerable
interest as moderate-temperature proton conductors. Structure dynamics during
proton conduction, particularly at grain boundaries, are crucial for stability
and proton transport in nanostructured PCCs. A common assumption is that PCCs
are structurally stable at low operating temperatures; however, material
polycrystallinity, absorption, and reactive operating conditions have so far
prevented verifying this assumption by nano resolved in-situ structure
measurements. Here, in an archetypal PCC BaZr0.8Y0.2O3-d the premise of
structural stability is demonstrated to be inaccurate at temperatures as low as
200 {\deg}C. Coherent X-ray diffraction on a nanostructured BaZr0.8Y0.2O3-d
sintered pellet is adapted to image in-situ three-dimensional structural
processes inside the constituent submicron grains in a humid nitrogen
atmosphere at 200 {\deg}C. Direct observation reveals non-equilibrium defect
generation and subsequent grain cracking on a timescale of hours, forming new,
otherwise energetically unfavorable facets in BaZr0.8Y0.2O3-d. Furthermore, the
structural rearrangements correlate with dynamic inhomogeneities of the lattice
constant within the grains, showing potential heterogeneous H+ transport. Our
results elucidate the mechanisms behind PCCs structural degradation, overturn
existing assumptions about the structure dynamics in PCCs, and fill a method
gap for further in-depth in-situ studies of the PCC nanostructure.
