MICROSCOPIC DYNAMICS OF ORTHO-TERPHENYL IN ITS POLYCRYSTALLINE AND GLASSY FORMS

The microscopic dynamics of polycrystalline ortho-terphenyl are investigated by means of the concurrent use of harmonic lattice dynamics calculations and Raman spectroscopy. In order to reproduce the spectra and thermodynamics of the crystal up to about 70 K, the motions of several internal modes are considered in detail. An empirical force field, able to account for the observed crystal structure, thermodynamics and Raman spectra up to such temperature, is then developed. Estimates of the anharmonic effects are derived from the temperature dependence of vibrational frequencies of those modes of collective or hybrid character (below 6 THz). The Raman spectra of the glass are finally discussed by comparison with the polycrystalline case. The implications of the present findings regarding studies on the glass transition of complex materials are discussed.