SURFACE LATTICE-DYNAMICS OF NIO-(001) BY INELASTIC HE-ATOM SCATTERING

High-resolution inelastic He atom scattering experiments, employing a time-of-flight (TOF) technique, have been carried out on NiO (001) in the [100] and [110] high symmetry directions. The targets were prepared by cleaving single-crystal NiO in air and then baking in vacuum. Nearly all of the experiments reported here were carried out at a target temperature of 575 K, which is above the Neel point (T(N) = 523 K), and with He atom wave vectors in the range of 7-9 angstrom-1 (25-45 meV). Because of these high temperatures, the peaks in the TOF spectra were found to be somewhat broader and the multiphonon background somewhat higher than in reported spectra from this instrument for alklai halide surfaces. Nonetheless, sufficient data were obtained to allow the determination of the Rayleigh wave almost to the Brillouin zone boundary. The results for this mode are found to be in good agreement with recently published, room-temperature electron energy loss spectroscopy experiments. In contrast, there is a substantial difference between these data and the dispersion data reported for Ni (001) with chemisorbed O atoms [the O-c(2 X 2) structure]. Experiments that were tried at low temperatures were unsuccessful due to the rapid accumulation of surface contaminants.