Neutron and Raman Scattering Studies of the Lattice and Methyl-Group Dynamics in Solid <i>p</i>-Xylene

Abstract Solid p-C6H4(CH3)2 and p-C6H4(CD3)2 have been studied over a temperature range from 10 K up to the melting point of ∼ 287 K by Raman- and Incoherent Inelastic Neutron-Scattering(IINS). The methyl torsion frequencies deduced from the Raman- and IINS-spectra are different because of the strong coupling of these modes with phonons and their dispersion in the Brillouin zone. A decrease in frequencies and a strong broadening, especially of the high frequency phonons where methyl torsions are predominant, have been observed in Raman- and IINS-spectra with increasing temperature. A lattice dynamics model based on the atom-atom potential set IVa of parameters given by Williams can, after some small modifications, satisfactorily explain the dynamics of a p-xylene crystal. Modified potential parameters better reproduce the experimental data and allow one to study the coupling of the internal methyl rotation with the lattice modes in more detail.

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