INTERNAL-FRICTION AND SOUND-WAVE VELOCITIES IN PURE C-60 FULLERITE

The temperature dependence of the attenuation and sound wave velocity of compacted pure C-60 fullerite is measured in the temperature interval from 100 to 300 K at a frequency of about 5 MHz. Two internal friction peaks are observed at 250 and 215 K. The conclusion is made that the internal friction peak at 250 K is connected with the structural phase transition and the peak at 215 K is due to the relaxation by reorientation of C-60 molecules in the stress field of ultrasonic waves. The magnitudes of the elastic moduli of C-60 fullerite are determined by sound velocity measurements. The results are compared with theoretical estimations of the elastic moduli and experimental data of other authors.