Oxygen vacancy induced surface stabilization: (110) terminated magnetite

Abstract

Scanning tunneling microscopy (STM) measurements of the (110) surface of magnetite showed the coexistence of two reconstructions: the known $(1\ifmmode\times\else\texttimes\fi{}3)$ row reconstruction and a surprising atomic structure of high complexity which occupies a small fraction of the surface. Oxygen vacancies on the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(110) B-terminated surface have previously been determined to be the most energetically favorable surface termination of those considered [Li et al., Surf. Sci. 601, 876 (2007)]. However, this study only investigated oxygen vacancies which were threefold coordinated. Here, first principles calculations indicate that twofold coordinated oxygen represents the most energetically stable oxygen vacancy on the B-terminated (110) surface of magnetite. STM simulations reveal that the structure that occupies a small fraction of the surface corresponds to this energetically favorable B-terminated ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(110) surface. The oxygen vacancies form an ordered array: Along the $[\overline{1}10]$ direction, every second twofold coordinated oxygen atom is vacant, and vacancies are separated by 6 \AA{}. In adjacent twofold coordinated oxygen rows, the vacancies are shifted in the $[\overline{1}10]$ direction by 3 \AA{}. Density functional theory calculations of the spin density distributions indicate that surface and subsurface octahedrally coordinated iron atoms are charge ordered. The charge ordering and existence of oxygen vacancies act to reduce the surface charge. However, other polarity compensation mechanisms may be at play to stabilize the surface.