Interaction of potassium with Fe O 111/ at elevated temperatures

The surface structures formed by the annealing of a potassium overlayer on Fe3O4 111.were investigated by low energy electron diffraction LEED., Auger electron spectroscopy AES.and scanning tunneling microscopy STM.. Annealing at 600–7008C in vacuum or 10y6 mbar of oxygen resulted in well-ordered surface structures depending on the amount of potassium pre-deposited. As the K coverage increased, the surface transformed gradually from a 4=4.to a 2=2., and then, to a 1=1.structure relative to the original Fe3O4 111.- 1=1.surface. At low coverage, the 4=4.structure was formed by a long-range modulation of the surface with an ;24 A°periodicity, which exhibited an internal 6 A°periodicity characteristic of the Fe3O4 111.- 1=1.surface. At mid-coverage, two sorts of 2=2.domains were observed, which were distinguished by the different diameter of protrusions forming an STM image. They were attributed to the different states of potassium in the top layer. These domains coexisted on the surface and were found in both oxidative and vacuum preparations. At high K coverage, the surface exhibited a 1=1.structure with a high density of vacancy defects. Auger depth-profile measurements confirmed the diffusion of potassium into the iron oxide bulk at elevated temperatures. The formation of a non-stoichiometric K2OrK2Fe22O34rFe3O4 111.interface with a gradually decreasing potassium concentration with depth has been suggested. q2000 Elsevier Science B.V. All rights reserved