Discontinuous volume change at the orientational-ordering transition in solid C sub 60

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