Investigation of the pressure dependence of the elastic constants of amorphous selenium in the vicinity of the glass transition

The 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 δC11,δP and δC44/δP increase rapidly as temperature is increased, a finding attributed to a combination of increasing vibrational anharmonicity as Tg is approached from below and to free-volume effects. At room temperature, substantially below Tg. the thermal Grüncisen parameter gammath is only 1·05 whereas the mean long-wavelength acoustic parameter gammael is 3·0. This arises because the intermolecular volume is much more compressible than the intramolecular volume: the Gruneisen parameters for purely internal modes of vibration are small whereas those for the external modes are larger.