SOUND-VELOCITY AND ATTENUATION IN SINGLE-CRYSTAL C60

We report the studies of the elasticity of C60 single crystals. For sublimed fcc crystals, Young's modulus has an 8% jump at the first-order transition at 260 K. At approximately 160 K there is a frequency-dependent elastic anomaly resulting from time-dependent stress relaxation. Comparison with rotation rates seen in NMR suggests that the dynamics below 260 K is more complex than jumps between equivalent molecular configurations. Solvent grown monoclinic crystals have a second-order transition at 242 K, while the 160-K glass transition remains unchanged.