Acoustic properties of borate glasses

The attenuation and velocity of ultrasonic waves of frequencies in the range from 10 MHz to 50 MHz have been measured in (M2O)0.14(B2O3)0.86 borate glasses (M = Li, Cs or Ag) as a function of temperature between 15 K and 300 K. Broad relaxation peaks characterize the acoustic attenuation at temperatures below 100 K and are paralleled by corresponding dispersive behaviors of the sound velocity. The velocity of sound waves decreases with increasing temperature in all the glasses, the decrease as the temperature is increased being larger in glasses containing cesium than in those with lithium or silver. Above 100 K the ultrasonic velocity is regulated by the vibrational anharmonicity and shows a nearly linear temperature dependence, whose slope scales with the size of the network modifier ion. The results suggest that the local motion of structural defects and the anharmonicity in borate glasses are strongly affected by the polarizing power of network modifier ions.