The Grüneisen parameter for bulk amorphous materials

Abstract

The longitudinal and shear velocities, and their pressure dependences, for variousbulk amorphous materials, including bulk metallic glasses and non-metallicglasses, have been measured by the ultrasonic pulse echo overlap methodunder hydrostatic pressure (maximum: 2 GPa) or uniaxial compression inambient conditions. The second- or third-order elastic constants and thelong-wavelength acoustic mode Grüneisen parameter are calculated. The resultsreveal that all metallic glasses have positive pressure dependences of both thelongitudinal and shear velocities, while for most non-metallic glasses both thelongitudinal and shear velocities decrease with increasing pressure. Thus,the Grüneisen parameters evaluated for the mean mode and shear modefor metallic glasses and non-metallic glasses have opposite signs. Thisindicates that the influences of the short-range-order structure and bonding,correlating closely with the atomic configurations in the various amorphousmaterials, play an important role in the properties determined by vibrationalanharmonicity. The negative and positive Grüneisen parameters exhibitmode softening and stiffness under high pressure, respectively. The resultsalso clearly demonstrate that the Grüneisen parameter is affected by thepressure derivative of the shear modulus, especially for non-metallic glasses.