Temperature-dependent surface structure and lattice dynamics of NiO(001)

Temperature-dependent surface relaxation and rumpling of NiO(001) were determined by high-resolution medium energy ion scattering. We found that the top-layer Ni plane is displaced toward the vacuum side relative to the top-layer O plane at temperature below 420 K. This is consistent with the ab initio calculation but contradicts the shell model calculation using pair potentials. However, at a temperature above 500 K (close to the Neel temperature), relative positions of the O plane to the Ni plane in the top and second layers are reversed. This result suggests that the surface structure is very sensitive to even a slight change of the bulk lattice. We also measured temperature dependent bulk and enhanced thermal vibrations (TVAs) of the top-layer atoms. The Debye temperatures for Ni and O estimated from the bulk TVAs of Ni and 0 using the simple Debye model are consistent with those derived from the heat capacity measurement. The bulk TVA gradually increases with increasing temperature, while the enhancement of the TVA of the top-layer Ni in the surface normal direction suddenly drops at 500 K. Such a dramatic change may be related to the reversal of the rumpling of the top layer. In contrast to alkali halide crystals, there are no correlations between the first nearest neighbor atoms in the [001] and [101] strings. The present result claims that pair potentials are not applicable to the NiO crystal which has, in part, covalent bonding.