Search for microscopic evidence for molecular level negative thermal expansion in fullerenes

We report the high-resolution far infrared vibrational properties of C-60 and endohedral Kr@C-60 fullerene as a function of temperature. Anomalous softening of the F-1u(1) mode (526 cm(-1)) is observed throughout the temperature range of investigation (300-10 K) suggesting that the fullerene cage may expand at low temperature in these molecular solids. To test this idea, we combine these results with temperature and pressure-dependent Raman, infrared, and Kr extended x-ray absorption fine structure data from the literature to provide a comprehensive view of local cage size effects. The results are consistent with a recent molecular dynamics study [Kwon , Phys. Rev. Lett. 92, 15901 (2004)] that predicts negative thermal expansion in carbon fullerenes.