Observation of the Sliding Mode in Incommensurate Intergrowth Compounds:Brillouin Scattering from the Inclusion Compound of Urea and Heptadecane

The anisotropy of the speeds of sound is reported for the incommensurate heptadecane/urea inclusion compound as measured by Brillouin scattering. In addition to the three normal acoustic modes, the results show the presence of a fourth sound velocity, corresponding to the so-called sliding mode (phason). A simple model for the elasticity of incommensurate crystals is used to discuss the polarizations of the sliding mode and the other acoustic mode