Interaction of potassium with Fe 3O 4(111) at elevated temperatures

Abstract

The surface structures formed by the annealing of a potassium overlayer on Fe 3O 4(111) were investigated by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Annealing at 600–700°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 (4×4) to a (2×2), and then, to a (1×1) structure relative to the original Fe 3O 4(111)-(1×1) surface. At low coverage, the (4×4) structure was formed by a long-range modulation of the surface with an ∼24 Å periodicity, which exhibited an internal 6 Å periodicity characteristic of the Fe 3O 4(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 K 2O/K 2Fe 22O 34/Fe 3O 4(111) interface with a gradually decreasing potassium concentration with depth has been suggested.