ULTRASONIC RELAXATIONS IN LANTHANIDE PHOSPHATE-GLASSES

The attenuation and velocity of ultrasonic waves of frequencies in the range of 10 to 90 MHz have been measured in La 2O3 P2O5 and Sm2O3 P2O5 glasses with high lanthanide concentrations as a function of temperature between 1.5 and 400 K. Two distinct features characterize the attenuation behavior: (i) a plateau at temperatures below 15 K and (ii) a broad high-temperature peak. The former feature is interpreted in terms of the phonon-assisted relaxation of two-level systems and the latter by assuming the existence of a distribution of thermally activated relaxing centers. For both these mechanisms the product of the deformation potential squared and the density of relaxing particles decreases with increasing lanthanide-ion concentration. This result, taken together with previous observations of the properties of oxide glasses, provides physical insight into the microscopic origin of the relaxation effects and suggests that the source of the low- and high-temperature attenuation mechanisms is the same. At temperatures below 100 K, the sound velocity, after the subtraction of the relaxation and anharmonic contributions, follows a linear law as predicted by the soft-potential model for the relaxation of soft harmonic oscillators. An encouraging agreement is obtained between the parameters regulating this mechanism and those determined from the acoustic attenuation plateau.