ASA 124th Meeting New Orleans 1992 October

3pPA4. Sound velocity and attenuation in crystalline C[sub 60].

X. D. Shi

Dept. of Phys., Univ. of Pennsylvania, Philadelphia, PA 19104

P. M. Chaikin

Princeton Univ., Princeton, NJ 08544

A. R. Kortan

AT&T Bell Labs., Murray Hill, NJ 07974

J. M. Williams

A. M. Kini

B. M. Savall

Argonne Natl. Lab., Argonne, IL 60439

A brief review is given on some of the most interesting structural and electrical properties in the C[sub 60] fullerenes. The elasticity study is reported on for C[sub 60] single crystals of different lattice symmetries. The temperature dependence of the sound velocity in the fcc crystals shows a discontinuous jump corresponding to about 8% increase in the modulus at an orientational ordering transition temperature T=260 K. This large change in the elastic modulus indicates a drastic change in the interaction mechanism between the molecules. At T~160 K, the sound velocity is observed to increase considerably accompanied by a profound peak in the sound attenuation whose position is frequency dependent. The 160 K anomaly is associated with relaxations of the C[sub 60] molecules to symmetry-inequivalent orientations in response to the stress field with the characteristic relaxation frequency following an activated temperature dependence and coinciding with the measurement frequency of about 20 kHz around 160 K. In the monoclinic crystals, the orientational ordering transition takes place at 242 K and is of second-order nature, while the 160 K freezing process remains unchanged.