Structural transition in SF₆ clusters

Abstract

SF⁶ clusters are produced during the free jet expansion of a Ne-SF⁶ mixture and studied by electron diffraction methods. Present experiments have been performed at constant nozzle diameter, stagnation pressure and stagnation temperature, adjustable parameters being the SF⁶ mole fraction X and the nozzle-to-skimmer distance x _s/d. It is possible to warm up SF₆ clusters by increasing either X, and thus the cluster size, or x _s/d, which makes them collide with background molecules downstream of the Mach disk. In both cases, cold clusters made of several hundreds of molecules experience a structural transition, similar to that observed in bulk material, from the triclinic to the bcc plastic structure. A molecular dynamics simulation accounts correctly for intermediate stages of the transition which are visible in experimental patterns. Contrary to bulk results, cluster results show that the structural transition occurs over some temperature range.