Structure of Monolayer and Multilayer Magnesium Chloride Films Grown on Pd(111)

The structures of magnesium chloride thin films grown on a Pd(111) surface have been studied using low-energy electron diffraction (LEED), temperature-programmed desorption, and Auger electron spectroscopy. At coverages slightly less than 1 monolayer a Pd(111)−(4×4)−MgCl 2 LEED pattern is observed by LEED that correlates with a coincident lattice match between Pd(111) and the (0001) or (001) face of either α or β close-packed MgCl 2, respectively. At full monolayer coverage, the existence of significant attractive adsorbate−substrate interactions is sufficient to induce a lattice compression of 0.36 Å in the MgCl 2 ionic solid in order to maximize the surface coverage. This more compressed structure gives rise to a Pd(111)−(√13×√13) R 13.9°−MgCl 2 overlayer. Upon deposition of thicker films the bulk structure is recovered, producing a simple MgCl 2 −(1×1) LEED pattern characteristic of the (001) orientation of crystalline MgCl 2 . This transition is consistent with the greater strength of interactions between successive MgCl 2 layers compared to those between the Pd(111) surface and the first MgCl 2 monolayer.