INTERNAL-FRICTION AND HYPERSONIC VELOCITY IN VITREOUS GERMANIA UNDER HIGH-PRESSURE

Measurements of internal friction and acoustic velocity at 25 GHz in v itreous germania (a-GeO2) from 50 K to 750 K and pressures to 4 GPa ar e reported. A maximum in internal friction and minimum in sound veloci ty are observed at about 1 GPa. Above 3 GPa, the acoustic properties b ecome increasingly irreversible. Results are discussed on the basis of relaxing entities residing in double-well potentials with a Gaussian distribution of barrier heights and a Lorentzian distribution in the a symmetry energy as in vitreous silica. The microscopic origin of the a nomalies might be structural changes which take place within small reg ions of the glass in analogy to phase transitions occurring in the cry stalline polymorphs under similar conditions. (C) 1998 Elsevier Scienc e B.V. All rights reserved.