When surface dynamics fakes symmetry: Oxygen on Rh(100) revisited

Heating a long-range ordered adsorbate phase beyond its stability temperature does not necessarily result in a disordered phase; it can also break up into heavily fluctuating ordered domains. Temporal and/or spatial averaging over these fluctuations may give the impression of both a wrong periodicity and false symmetry elements such as nonexisting glide planes or rotational axes of incorrect order. This can happen in real space and reciprocal space, and also below liquid-nitrogen temperature, so that the true nature of the phase might remain undetected. We demonstrate this scenario at the catalytically active Rh(100) surface covered by 1 2 monolayer of oxygen, using quantitative low-energy electron diffraction, variable-temperature scanning tunneling microscopy, and density functional theory. Using the example of CO adsorption, we show that local symmetry can have a decisive influence on the binding energy and thus the chemical reactivity.