Quantitative Evaluation of Anharmonic and Disorder Effects on Glassy Dynamics

Abstract

Measurements of the pressure dependence at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}80\mathrm{K}$ of the vibrational spectrum of a polymorphic material in its amorphous (G), orientationally disordered crystal (ODC), and ordered crystal (C) phases has permitted the separation of orientational and site disorder effects on Gr\"uneisen parameters used here, pars pro toto, as a measure of anharmonicity. The greater anharmonicity found for G compared to ODC is consistent with their differences in specific heat, while the very similar and strong vibrational attenuation in the two glassy phases indicates that phonon scattering for $T$ above the thermal conductivity plateau arises mainly from orientational disorder of the molecules.