Influence of Ca and K on the reconstruction of the Fe3O4(0 0 1) surface

We have studied the thermally activated segregation of Ca and K impurities at the Fe3O4(0 0 1) surface in ultrahigh vacuum. Using Auger electron spectroscopy, we have established that the segregation process is accompanied by a depletion of oxygen from the Fe3O4(0 0 1) surface. Atomically resolved STM images show that a p(1 × 4) missing row reconstruction is typically formed on the surface. We propose that this reconstruction is driven by the accommodation of Ca at the surface, resulting in the formation of a Ca1−xFe2+xO4 surface phase. The missing row structure, due to oxygen desorption, provides an increased number of Ca binding sites compared to the flat Fe3O4(0 0 1) surface. The accommodation of Ca and the desorption of oxygen from the surface contribute to a reduction in the surface energy through charge compensation of the polar Fe3O4(0 0 1) surface.