Order-disorder phase transition of the Cu(001) surface under equilibrium oxygen pressure

In situ surface x-ray scattering is used to determine the stable oxygen-induced surface structure on Cu(001) at high temperatures and under equilibrium oxygen pressure. The structure is composed of a c(2x2)-O adlayer atop a randomly 3/4-filled Cu layer for temperatures between 473 and 1000 K. Below 473 K, the vacancies in the topmost Cu layer order to form a (2 root 2x root 2)R45 degrees missing row structure. This order-disorder transition is confirmed by first-principles calculations.