  Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in
technology and environmental studies, as it is closely linked to both catalytic
activity and material degradation. The atomic-scale details are mostly unknown,
however, and are often ascribed to poorly defined defects in the crystal
lattice. Here we show that even thermodynamically stable, well-ordered surfaces
can be surprisingly reactive. Specifically, we show that all the 3-fold
coordinated lattice oxygen atoms on a defect-free single-crystalline r-cut
(1-102) surface of hematite ({\alpha}-Fe2O3) are exchanged with oxygen from
surrounding water vapor within minutes at temperatures below 70 {\deg}C, while
the atomic-scale surface structure is unperturbed by the process. A similar
behavior is observed after liquid water exposure, but the experimental data
clearly show most of the exchange happens during desorption of the final
monolayer, not during immersion. Density functional theory computations show
that the exchange can happen during on-surface diffusion, where the cost of the
lattice oxygen extraction is compensated by the stability of an HO-HOH-OH
complex. Such insights into lattice oxygen stability are highly relevant for
many research fields ranging from catalysis and hydrogen production to
geochemistry and paleoclimatology.
