Thermal effects on the dynamics and electronic states ofNa3C60

Raman-scattering and ultraviolet-photoemission measurements on ${\mathrm{Na}}_3$ ${\mathrm{C}}_{60}$ thin films indicate a transition at elevated temperatures to a system with increased metallic character. This is observed by a substantial increase in line widths and peak shifts of selected Raman ${\mathit{H}}_{\mathit{g}}$(i) modes accompanied by an increase in the density of states at the Fermi energy as measured by photoemission. Changes in these spectra with temperature, as well as differences with ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$, suggest that low-temperature nonmetallic behavior is due to octahedral Na site disorder.