The reconstruction of nickel and rhodium (001) surfaces upon carbon, nitrogen or oxygen adsorptions

Nickel and rhodium (001) surfaces display a similar tin terms of scanning t unneling microscopy images) clock reconstruction when half a monolayer of C -Ni, N-Ni or O-Rh is adsorbed; no reconstruction is observed instead for O- Ni. Adsorbate atoms sit at the center of the black squares of a chess-board , c(2 x 2), pattern and two different reconstructions are actually compatib le with the observed STM images - showing a (2 x 2)p4g pattern according to whether a rotation of the black or white squares occurs. We report on a fi rst-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 all available experimental info rmation and shed new light on the mechanisms responsible for the reconstruc tions. We show that the same substrate may display different reconstruction s (or no reconstruction) upon adsorption of different atomic species, depen ding on the relative importance of the chemical and steric factors that det ermine the reconstruction. (C) 1999 Published by Elsevier Science B.V. All rights reserved.