Interaction of potassium with Fe3O4(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 600-700 degrees C in vacuum or 10(-6) 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 x 4) to a (2 x 2), and then, to a (1 x 1) structure relative to the original Fe3O4(111)-(1 x 1) surface. At low coverage, the (4 x 4) structure was formed by a long-range modulation of the surface with an similar to 24 Angstrom periodicity, which exhibited an internal 6 Angstrom periodicity characteristic of the Fe3O4(111)-(1 x 1) surface. At mid-coverage, two sorts of (2 x 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. Ar high K coverage, the surface exhibited a (1 x 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 K2O/K2Fe22O34/Fe3O4(111) interface with a gradually decreasing potassium concentration with depth has been suggested. (C) 2000 Elsevier Science B.V. All rights reserved.