The adsorption of gas atoms on Cr surfaces was known to lead to unusually large substrate relaxations, and the adsorbate-induced formation of surface vacancies had even been reported for O adsorption. An investigation was made here of the structure and electronic properties of clean or adsorbate-covered Cr(100) surfaces by using first-principles density-functional calculations. The large outward relaxation which was observed upon the adsorption of C, N or O was shown to be due to the depletion of bonding orbitals of the substrate; as caused by electron donation to the adsorbate. With increasing numbers of p-electrons of the adsorbed atom, back-donation effects led to an incipient filling of antibonding in-plane orbitals of the substrate. In the case of O on Cr(100), this effect was sufficiently strong to promote the formation of surface vacancies.

Adsorbate-Induced Vacancy Formation and Substrate Relaxation on Cr(100). A.Eichler, J.Hafner: Physical Review B, 2000, 62[8], 5163-7