Using scanning tunneling microscopy, low-energy ion scattering, and quantit
ative low-energy electron diffraction, we find about 17% metal vacancies on
the oxygen-covered Cr(100) surface. The oxygen atoms occupy all the hollow
sites of the first layer, including those neighboring a Cr vacancy. We arg
ue that the vacancy formation is energetically favored and not caused by st
ress but by electronic effects.