Structural and spin state transition in the polar NiO(1 1 1) surface

Abstract Geometric and electronic structure of surfaces is of fundamental importance for the understanding of the physical and chemical phenomena of transition metal oxides. Here we report a structural and spin state transition in the NiO(1 1 1) surface. Combining aberration-corrected transmission electron microscopy and first-principles calculations, we show that the surface can be stabilized in a bulk-truncated structure and a subsurface tetrahedral reconstructed one, both with a (1×1) periodicity and the hydroxylation ruled out. The transition from the bulk-truncation to the tetrahedral reconstruction is accomplished by the shift of the subsurface Ni atoms from octahedral to tetrahedral position, accompanied by a spin transition from a low-spin state to a high-spin state.