Abstract
Hydrostatic pressure dependences of the velocities of longitudinal and shear ultrasonic waves propagated in amorphous selenium have been measured as a function of temperature through the glass transition region. Both dC11/dP and dC44/dP increase rapidly as Tg is approached. This finding is attributed to a combination of increasing vibrational anharmonicity as the temperature rises as well as to free volume effects. At room temperature, well below Tg, the thermal Gruneisen parameter γth is only 1.05 whereas the mean long wavelength acoustic parameter γel is 3.0. In this molecular glass, the intermolecular volume is much more compressible than the intramolecular volume: the Gruneisen parameters for purely internal modes of vibrational are small, whereas those for the external modes are larger.
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