Formation and stability of Fe-rich terminations of the Fe₃O₄(001) surface

Abstract

Understanding how the structure of iron oxide surfaces varies with their environment is essential for rationalizing their role in (geo-)chemistry and optimizing their application in modern technologies. In this paper, we create Fe-rich terminations of Fe₃O₄(001) by depositing iron directly onto the ‘subsurface cation vacancy’-reconstructed surface, which is the most stable surface under ultrahigh vacuum conditions. Scanning tunneling microscopy and x-ray photoelectron spectroscopy data reveal that the excess iron is initially accommodated as two-fold coordinated adatoms and later incorporates into the subsurface cation vacancies. As the coverage increases, small patches of the octahedral pair termination (also known as the ‘Fe dimer’ termination) nucleate, eventually covering the entire surface after the deposition of 2 iron atoms per (√2×√2)R45° unit cell. This conclusion effectively rules out some existing models for the termination and provides support for the model proposed by Rustad et al (Surface Science 432, L583-L588, 1999), highlighting the need for further theoretical work to complete the Fe₃O₄(001) surface phase diagram. The octahedral pair termination is found to be unstable above 523 K and upon exposure to molecular O₂ because the excess iron atoms agglomerate to form small FeOₓ clusters.