Structural Aspects of Nanocrystals of Transition-Metal Hexafluorides

Abstract

Experiments carried out under systematically changing conditions were performed to generate nanocrystals of MoF6 and WF6 in supersonic expansions of the vapor. Results were monitored by electron diffraction. Under warmer expansion conditions the bcc crystals produced rapidly transformed to a metastable monoclinic phase unknown in the bulk, whereas nucleation at colder temperatures led directly to the orthorhombic phase generally considered to be the stable low-temperature allotrope. When nucleation was postponed until the flow was very cold, the orthorhombic diffraction pattern appeared but was mixed with a pattern that could not be identified with any known phase of any hexahalide. Although the evidence is not absolutely conclusive, we believe that a new phase was formed. If this is true, it is an open question whether the phase is another metastable allotrope owing its existence to the kinetics rather than the thermodynamics of formation or whether it, rather than the orthorhombic structure, constitutes the actual low-temperature phase of transition-metal hexafluorides. Cell constants of the recognizable nanocrystals were determined to ascertain whether size effects or effects of the kinetics of growth play a role. Lattice constants of 10 nm crystals of the orthorhombic phase formed in microseconds were indistinguishable from those of the bulk. Variations among the shapes of orthorhombic cells of the various hexafluorides were analyzed and found to be related to the temperature, the bond polarity, and the bond length.

Diffraction of Trapped(CsI)nCs+: The Appearance of Bulk Structure

“…Electron diffraction studies have been performed previously on clusters in supersonic beams [10][11][12][13] composed of a distribution of cluster sizes and internal energies. In the measurements described here, it is precisely the ability to mass select and control the cluster temperature which provides the opportunity to observe and interpret the details of size dependent structure.…”