The adsorption of Ni, Co, Mn, Ti, and Zr at the (√2 × √2)R45°-reconstructed Fe3O4(001) surface was studied by scanning tunneling microscopy (STM), X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), low energy electron diffraction (LEED), and density functional theory (DFT). Following deposition at room temperature, metals are either adsorbed as isolated adatoms, or fill the subsurface cation vacancy sites responsible for the (√2 × √2)R45° reconstruction. Both configurations coexist, but the ratio of adatoms to incorporated atoms depends on the metal; Ni prefers the adatom configuration, Co and Mn form adatoms and incorporated atoms in similar numbers, and Ti and Zr are almost fully incorporated. With mild annealing, all adatoms transition to the incorporated cation configuration. At high coverage, the (√2 × √2)R45° reconstruction is lifted because all subsurface cation vacancies become occupied with metal atoms, a (1 × 1) LEED pattern is observed. DFT+U calculations for the extreme cases, Ni and Ti, confirm the energetic preference for incorporation, with calculated oxidation states in good agreement with photoemission experiments. Because the site preference is analogous to bulk ferrite (XFe2O4) compounds, similar behavior is likely to be typical for elements forming solid solution with Fe3O4.
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Corresponding author: Gareth Parkinson (parkinson).