Geometry of α‑Cr₂O₃(0001) as a Function of H₂O Partial Pressure

Surface X-ray diffraction has been employed to elucidate the surface structure of α-Cr₂O₃(0001) as a function of water partial pressure at room temperature. In ultra high vacuum, following exposure to ∼2000 Langmuir of H₂O, the surface is found to be terminated by a partially occupied double layer of chromium atoms. No evidence of adsorbed OH/H₂O is found, which is likely due to either adsorption at minority sites, or X-ray induced desorption. At a water partial pressure of ∼30 mbar, a single OH/H₂O species is found to be bound atop each surface Cr atom. This adsorption geometry does not agree with that predicted by ab initio calculations, which may be a result of some differences between the experimental conditions and those modeled.