Stressing Pd atoms: Initial oxidation of the Pd(1Â 1Â 0) surface

We have investigated the oxygen induced structures of the Pd(1 1 0) surface in the pressure range of 10-5-10-3 mbar of oxygen, at a sample temperature of around 300 °C. These structures, denoted as “(7×3)” and “(9×3)”, are studied in detail by the use of a combination of low-energy electron diffraction, scanning tunneling microscopy, high-resolution core level spectroscopy, and ab-initio simulations. Based on our data a model is proposed for these structures containing segments of Pd atoms in the [11¯0] direction, in which the Pd rows are decorated by O atoms in a zig-zag pattern. The segments are periodically separated by displaced Pd atoms. Density functional theory calculations show that the displacements reduce the oxygen induced stress significantly, as compared to a structure with no displacements. The calculations also suggest that the new structures are stabilized by domain formation.