(SF6)(N) CLUSTERS, THAN-OR-SIMILAR-TO-N-LESS-THAN-OR-SIMILAR-TO-3000,PRODUCED IN A SF6-EXPANSION - SIZE, TEMPERATURE, AND SOLID-PHASE TRANSITION(NE GAS)

In this paper, the phase behavior of SF6 clusters is examined experime ntally and is discussed in the context of the previous work. SF6 clust ers made of 100 to 3000 molecules are produced in a free jet expansion of a Ne+SF6 mixture. Cluster structures are identified by means of el ectron diffraction methods and ascertained by molecular dynamics (MD) simulations. On warming up the clusters, diffraction patterns display the transition from the monoclinic (low temperature) to the body cente red cubic (high temperature) bulk structure, finite size effects appea ring in the form of intermediate patterns that correspond to neither s tructure. MD simulations have shown that these intermediate patterns a re due to a progressive rearrangement of the cluster surface prior to the cluster core transition, a process which leads to the observed tem perature spread of the transformation. Taking advantage of the sensiti vity of diffraction patterns to cluster temperature, SF6 clusters are used to probe the free jet expansion, particularly the cooling efficie ncy of the carrier gas and the warming effect caused by the crossing o f the frontal shock wave. It is found that upon increasing the SF6 mol e fraction, clusters become larger and warmer, the high-temperature st ructure being achieved when the expanding mixture is nearly saturated in SF6, which corresponds to a maximum cluster size. When cold cluster s are allowed to cross the frontal shock wave, they warm up and acquir e the cubic structure, without any appreciable evaporation. Using line height measurements in the cubic patterns, it is shown that the varia tion of the Debye-Waller factor, in a large range of sizes, is mainly due to a size effect. Finally, the temperature at which the transition to the cubic structure occurs is found to be constant for clusters ma de of more than about 1300 molecules; however, it decreases when the c lusters get smaller. This result has been confirmed by recent molecula r dynamics simulations. (C) 1995 American Institute of Physics.