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

We report the high-resolution far infrared vibrational properties of C60 and endohedral Kr@C60 fullerene as a function of temperature. Anomalous softening of the F1u(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 et al., Phys. Rev. Lett. 92, 15901 (2004)] that predicts negative thermal expansion in carbon fullerenes.