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 coadsorpti on 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 fo ur O atoms are displaced by 0.35 +/- 0.08 A from the centers of the hollows , and the average O-Ni distance is 1.92 +/- 0.17 Angstrom. The K-O distance is 2.93 +/- 0.15 Angstrom and the K-Ni distance is 3.33 +/- 0.12 Angstrom. We propose that the primary driving force for the creation of the vacancie s is the reduction of energy when the K atoms move closer to the surface. A t a later stage of oxidation, these vacancies may facilitate the formation of NiO by helping to alleviate the strain in the lattice mismatch between t he NiO structure and Ni(100). O 2000 Elsevier Science B.V. All rights reser ved.