  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.
