Strained c(4 x 2) CoO(100)-like monolayer on Pd(100): Experiment and theory

We report on an interface-stabilized strained c(4 x 2) phase formed by cobalt oxide on Pd(100). The structural details and electronic properties of this oxide monolayer are elucidated by combination of scanning tunneling microscopy data, high resolution electron energy loss spectroscopy measurements and density functional theory. The c(4 x 2) periodicity is shown to arise from a rhombic array of Co vacancies, which form in a pseudomorphic CoO(100) monolayer to partially compensate for the compressive strain associated with the large lattice mismatch (similar to 9.5%) between cobalt monoxide and the substrate. Deviation from the perfect 1:1 stoichiometry thus appears to offer a common and stable mechanism for strain release in Pd(100) supported monolayers of transition metal rocksalt monoxides of the first transition series, as very similar metal-deficient c(4 x 2) structures have been previously found for nickel and manganese oxides on the same substrate. (C) 2009 Elsevier B.V. All rights reserved.