Adsorbate-induced vacancy formation and substrate relaxation on Cr(100)

The adsorption of gas atoms on Cr surfaces has been shown to lead to unusually large substrate relaxations; for O adsorption even an adsorbate-induced formation of surface vacancies has been reported. We have investigated the structure and electronic properties of clean and adsorbate-covered Cr(100) surfaces using first-principles density-functional calculations. The large outward relaxation observed on adsorption of C, N, or O is shown to be due to the depletion of bonding orbitals of the substrate caused by electron donation to the adsorbate. With an increasing number of p electrons of the adsorbed atom, back-donation effects lead to an incipient filling of antibonding in-plane orbitals of the substrate. For O:Cr(100) this effect is strong enough to promote the formation of surface vacancies.