Discontinuous volume change at the orientational-ordering transition in solid C60.

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\ifmmode\pm\else\textpm\fi{}0.004 \AA{} on heating, indicating a strongly first-order transition. The average isobaric volume thermal-expansion coefficient both above and below the transition is 6.2\ifmmode\pm\else\textpm\fi{}0.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$. We observe phase coexistence over a 5-K range, but little if any hysteresis.