Lattice dynamics of CuGeO 3 : Inelastic neutron scattering and model calculations

The lattice dynamics in CuGeO 3 has been analyzed by the combination of inelastic neutron scattering studies and lattice-dynamical model calculations. We report on an almost complete set of dispersion curves along the three orthorhombic directions and along [101]. Some modes exhibit rather low phonon frequencies in relation either with the almost layered character of the crystal structure or with an instability of the CuO 2 ribbons and GeO 4 chains against twisting. The dispersion of branches associated with the modes directly involved in the spin-Peierls transition allows to explain the particular propagation vector of the structural distortion in the dimerized phase.