Quantitative evaluation of anharmonic and disorder effects on glassy dynamics

Measurements of the pressure dependence at T = 80 K of the vibrational spec trum of a polymorphic material in its amorphous (G), orientationally disord ered crystal (ODC), and ordered crystal (C) phases has permitted the separa tion of orientational and site disorder effects on Gruneisen parameters use d here, pars pro toto, as a measure of anharmonicity. The greater anharmoni city found for G compared to ODC is consistent with their differences in sp ecific heat, while the very similar and strong vibrational attenuation in t he two glassy phases indicates that phonon scattering for T above the therm al conductivity plateau ar ises mainly from orientational disorder of the m olecules. [S0031-9007(98)08355-0].