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 La2O3-P2O5 and Sm2O3-P2O5 glasses with high lanthanide concentrations as a function of temperat ure between 1.5 and 400 K. Two distinct features characterize the atte nuation behavior: (i) a plateau at temperatures below 15 K and (ii) a broad high-temperature peak. The former feature is interpreted in term s of the phonon-assisted relaxation of two-level systems and the latte r by assuming the existence of a distribution of thermally activated r elaxing centers. For both these mechanisms the product of the deformat ion potential squared and the density of relaxing particles decreases with increasing lanthanide-ion concentration. This result, taken toget her with previous observations of the properties of oxide glasses, pro vides physical insight into the microscopic origin of the relaxation e ffects and suggests that the source of the low- and high-temperature a ttenuation mechanisms is the same. At temperatures below 100 K, the so und velocity, after the subtraction of the relaxation and anharmonic c ontributions, 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.