Abstract
Using density-functional theory (DFT) we calculate the Gibbs free energy to determine the lowest-energy structure of a 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 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 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 non-stoichiometric terminations is a more general phenomenon on transition metal oxide surfaces.
Highlights
Density-functional theoryDFTis often argued to be a zero-temperature, zero-pressure technique
When we aim to describe experiments that are conducted at constant pressure and temperature, the appropriate thermodynamic potential to consider is the Gibbs free energy G(T,p)
A polar termination is traditionally not considered to be stable within the framework of electrostatic arguments based on the ionic model of oxides
Summary
Density-functional theoryDFTis often argued to be a zero-temperature, zero-pressure technique. Extrapolation of the low-pressure results to technical processes taking place at ambient atmosphere is often not possible, which has been coined with buzz words like pressure and materials gapsee, e.g., the discussion in Stampfl et al.[1] and references therein. Trying to bridge these gaps, one needs to determine the equilibrium composition and geometry of a surface in contact with a given environment at finite temperature and pressure. If DFT total energies enter in a suitable way into a calculation of G(T, p) for a material surface, ab initio thermodynamics is the result, and the predictive power of the first-principles technique is extended to a more relevant temperature and pressure range
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