The lattice dynamics of a dipolar disordered crystal of 2-3-dimethylanthracene

Abstract By the method of incoherent and coherent inelastic neutron scattering the amplitude weighted phonon spectra ḡ(ω) as well as phonon dispersion curves of 2-3-dimethylanthracene (2-3-DMA, C 16 H 14 ) were measured. Samples with different degrees of deuteration were used for a better understanding of the lattice vibrations. Crystalline 2-3-DMA has two molecules in the unit cell, showing dipolar disorder in orientation. This disorder causes that the widths of the phonon lines are very broad, even at low temperatures. The phonon energies are explained by a calculation of the lattice dynamics, using an “6-exp” potential between atoms of different molecules, to describe the forces between them and taking into account the internal degrees of freedom in the molecules.