Composition, structure, and stability of RuO 2 ( 110 ) as a function of oxygen pressure

TLDR: In this article, the Gibbs free energy was calculated to determine the lowest energy structure of a transition metal oxide surface in thermodynamic equilibrium with an oxygen-rich environment, and it was shown that the commonly assumed stoichiometric termination is only favorable at low oxygen chemical potentials, i.e., low pressures and/or high temperatures.

Abstract: Using density-functional theory we calculate the Gibbs free energy to determine the lowest-energy structure of a ${\mathrm{RuO}}_{2}(110)$ surface in thermodynamic equilibrium with an oxygen-rich environment. The traditionally assumed stoichiometric termination is only found to be favorable at low oxygen chemical potentials, i.e., low pressures and/or high temperatures. At a realistic O pressure, the surface is predicted to contain additional terminal O atoms. Although this O excess defines a so-called polar surface, we show that the prevalent ionic model, that dismisses such terminations on electrostatic grounds, is of little validity for ${\mathrm{RuO}}_{2}(110).$ Together with analogous results obtained previously at the (0001) surface of corundum-structured oxides, these findings on (110) rutile indicate that the stability of nonstoichiometric terminations is a more general phenomenon of transition metal oxide surfaces.