Discontinuous volume change at the orientational-ordering transition in solidC60

Abstract

X-ray and neutron-diffraction measurements have been used to study the evolution of the lattice parameter of solid ${\mathrm{C}}_{60}$ through the orientational-ordering transition. The lattice parameter jumps by +0.044±0.004 Å on heating, indicating a strongly first-order transition. The average isobaric volume thermal-expansion coefficient both above and below the transition is 6.2±0.2×${10}^{\mathrm{-}5}$ ${\mathrm{K}}^{\mathrm{-}1}$. We observe phase coexistence over a 5-K range, but little if any hysteresis.