The reconstructions of Ni and Rh (001) surfaces upon Carbon, Nitrogen and Oxygen adsorption

Nickel and Rhodium (001) surfaces display a similar, as from STM images, "clock reconstruction" when half a monolayer of C/Ni, N/Ni or O/Rh is adsorbed; no reconstruction is observed for O/Ni; no experimental data are available for C/Rh and N/Rh. In the unreconstructed surface, adsorbate atoms sit at the center of the black squares of a chess-board, c(2× 2), pattern and two different reconstructions are actually compatible with the observed STM images, showing a (2× 2)p4g pattern, according to whether a rotation of the black or white squares is present. We report on a first-principles study of the structure of X/Ni(001) and X/Rh(001) surfaces (X=C,N,O) at half a monolayer coverage, performed using density-functional theory. Our findings are in agreement with available experimental data and show that the same substrate may display different reconstructions, or no reconstruction, according to the adsorbed species, while the same species may induce different reconstructions on different substrates. The different factors at the origin of this behavior will be discussed.