Phonon anharmonicity near the melting point and the glass transition

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Abstract The vibrational anharmonicity of long-wavelength acoustic phonons has been determined for vitreous selenium in the vicinity of its glass transition and for monocrystalline indium as it approaches its melting point. The experimental method used was to measure the hydrostatic pressure dependences of ultrasonic wave velocities as a function of temperature. For amorphous selenium, both dC 11/dP and dC 44/dP increase substantially as the temperature is raised towards T g, indicating that vibrational anharmonicity rises as this glass nears T g. Well below T g the Grüneisen parameters for purely internal modes of vibration are small, whereas those for external modes are larger; this is because the inter-molecular volume is much more compressible than the intramolecular volume. The experimental results for indium provide the first complete set of the pressure derivatives of elastic constants of a material up to its melting point. The acoustic-mode Grüneisen parameters in the long-wavelength limit show that there is a substantial increase in vibrational anharmonicity as T m is approached. Finally, measurements of the wave-vector dependences of the transverse acoustic phonon energy widths are reported for single-crystal lead up to T m. It has been found that there is a change from a zero-sound collision-free mode of high-frequency phonon propagation to a first-sound collision-dominated regime at about 500–550 K. Hence the phonons which have wavelengths on the scale of the interatomic spacing are heavily damped in the crystal prior to melting.