By using scanning tunnelling microscopy, low-energy ion scattering, and quantitative low-energy electron diffraction techniques, it was found that there were about 17% of metal vacancies on an O monolayer-covered Cr(100) surface. The Cr vacancies were found after both the room-temperature adsorption of O2, and the segregation of O at high temperatures. The O atoms occupied all of the hollow sites of the first layer, including those neighboring a Cr vacancy. Although neither the atomic radii, nor a comparison with bulk structures or in-plane relaxations of Cr atoms, suggested that compressive stress was a driving force for the vacancy formation, electronic effects that were similar to those in bulk NaCl-type transition-metal compounds were proposed to be a reason for the vacancy formation.

Oxygen-Induced Vacancy Formation on a Metal Surface. M.Schmid, G.Leonardelli, M.Sporn, E.Platzgummer, W.Hebenstreit, M.Pinczolits, P.Varga: Physical Review Letters, 1999, 82[2], 355-8