Structural and magnetic transition in NiO(111) surface

We report an experimental and theoretical analysis of the NiO(111) surfaces combining aberration corrected TEM and first-principles calculations. An unreconstructed O-terminated (1×1) surface and a reconstructed O-terminated surface (with the topmost Ni in tetrahedral sites, a wurtzite-like surface stacking) have been revealed. Here, the two surfaces are shown to be stabilized by charge compensation, arising from changing the valence state of the topmost Ni²⁺ into Ni³⁺ ions and providing the additional positive charges to the surface. In addition, it is found that the main difference between the two surface structures is the movement of the topmost Ni atoms from octahedral position to tetrahedral position, coupling to a spin transition from the low-spin state (Ni³⁺-LS) to the high-spin state (Ni³⁺-HS).