Low-energy electron diffraction study of the surface geometry of Ni(100)-(3 x 3)-K + 40

A dynamical LEED study of the (3 x 3) structure which forms upon coadsorption of K and O on Ni(100) indicates that this structure is comprised of one K atom and four O atoms per (3 x 3) unit cell. The K atom is adsorbed in a substitutional site, and surrounded by four O atoms in hollow sites. The four O atoms are displaced by 0.35 plus/minus 0.08 A from the centers of the hollows, and the average O-Ni distance is 1.92 plus/minus 0.17 A. The K-O distance is 2.93 plus/minus 0.15 A and the K-Ni distance is 3.33 plus/minus 0.12 A. We propose that the primary driving force for the creation of the vacancies is the reduction of energy when the K atoms move closer to the surface. At a later stage of oxidation, these vacancies may facilitate the formation of NiO by helping to alleviate the strain in the lattice mismatch between the NiO structure and Ni(100).