Stressing Pd atoms: Initial oxidation of the Pd(1 1 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 [ 1 1 ¯ 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.