THE LATTICE-DYNAMICS OF A DIPOLAR DISORDERED CRYSTAL OF 2-3-DIMETHYLANTHRACENE

By the method of incoherent and coherent inelastic neutron scattering the amplitude weighted phonon spectra (g) over bar(omega) as well as p honon dispersion curves of 2-3-dimethylanthracene (2-3-DMA, C16H14) we re measured. Samples with different degrees of deuteration were used f or a better understanding of the lattice vibrations. Crystalline 2-3-D MA has two molecules in the unit cell, showing dipolar disorder in ori entation. This disorder causes that the widths of the phonon lines are very broad, even at low temperatures. The phonon energies are explain ed by a calculation of the lattice dynamics, using an ''6-exp'' potent ial between atoms of different molecules, to describe the forces betwe en them and taking into account the internal degrees of freedom in the molecules.