Order-disorder transition in monoclinic sulfur: a precise structural study by high-resolution neutron powder diffraction

High-resolution neutron powder diffraction has been used in order to characterize the order-disorder transition in monoclinic cyclo-octasulphur. Rapid data collection and the novel use of geometrically constrained refinements has enabled a direct and precise determination of the order parameter, based on molecular site occupancies, to be made. The transition is critical and continuous; with a transition temperature, T-c = 198.4 ( 3) K, and a critical exponent, beta = 0.28 (3), which is indicative of three-dimensional ordering. Difficulties encountered as a consequence of the low thermal conductivity of the sample are discussed.