In Search of Microscopic Evidence for Molecular Level Negative Thermal Expansion in Fullerenes

TL;DR

This study investigates the vibrational properties of C60 and Kr@C60 fullerenes at various temperatures, providing evidence that supports the existence of negative thermal expansion at the molecular level in these materials.

Contribution

It offers the first high-resolution infrared vibrational data combined with literature analysis to support negative thermal expansion in fullerenes, aligning with recent molecular dynamics predictions.

Findings

Anomalous softening of vibrational modes at low temperatures

Evidence of cage expansion at low temperature in fullerenes

Support for molecular-level negative thermal expansion in carbon fullerenes

Abstract

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.