The surface phonon dispersion of clean (100) surfaces was measured by means of high resolution electron energy loss spectroscopy. Eight vibrational modes, most of which were located in the energy gaps between the bulk phonon bands, were detected. Lattice dynamics calculations of a slab-shaped crystal based upon a force constant model reproduced such experimental dispersion curves, which indicated a 25% increase in the force constant of inter-octahedral boron-boron bonds at the surface, and a 25-50% decrease in the force constants of intra-octahedral boron-boron bonds at the surface. These changes, together with an analysis of the bulk phonon structure of metal hexaborides, strongly suggested that the boron octahedra at the surface were expanded by about 0.004nm parallel to the surface.

Surface Phonons of LaB6(100): Deformation of Boron Octahedra at the Surface. Tadaki, N., Kazuo, K., Yoshitaka, I., Chuhei, O., Otani, S.: Surface Science, 1993, 290[3], 436-44. See also: Surface Science, 1993, 287-288[1], 391-5