Lattice-dynamical calculations in the rigid-body approximation were performed for the dimer and tetramer phases of N-methyl-N-ethyl-morpholinium bis(tetracyanoquinodimethane) which are the phases above and below the spin-Peierls transition, respectively. A suitable choice of nonbonded atom-atom potentials gave a good agreement between the calculated phonon frequencies and the absorption-line frequencies in the far-infrared spectra. The polarization properties of the absorption lines were also consistent with the calculated phonon modes. The far-infrared lines assigned to lattice phonons include the 150.5 and 130.5 ${\mathrm{cm}}^{\mathrm{-}1}$ lines in the dimer phase and the corresponding lines at 156 and 126 ${\mathrm{cm}}^{\mathrm{-}1}$ in the tetramer phase. The frequency shifts upon the spin-Peierls transition are reproduced in the calculations and ascribed to the lattice distortion caused by the transition.

• Received 11 December 1987

DOI:https://doi.org/10.1103/PhysRevB.37.7691