Surface phonon dispersion curves of the LaB₆(111) surface

Energy dispersion curves of the surface phonons of the LaB₆ (111) surface have been measured by using high resolution electron energy loss spectroscopy. We found two large loss peaks and three small loss peaks in the specular spectra and several branches in the off-specular spectra. On the basis of lattice dynamical calculations using bulk parameters, we have obtained the theoretical phonon dispersion curves of the unrelaxed surface. One vibrational mode appeared above the bulk phonon bands, which indicates stiffening of the B-B bonds of the surface octahedra.