STRUCTURAL ASPECTS OF NANOCRYSTALS OF TRANSITION-METAL HEXAFLUORIDES

Experiments carried out under systematically changing conditions were performed to generate nanocrystals of MoF6 and WF6 in supersonic expan sions of the vapor. Results were monitored by electron diffraction. Un der warmer expansion conditions the bcc crystals produced rapidly tran sformed to a metastable monoclinic phase unknown in the bulk, whereas nucleation at colder temperatures led directly to the orthorhombic pha se generally considered to be the stable low-temperature allotrope. Wh en nucleation was postponed until the flow was very cold, the orthorho mbic diffraction pattern appeared but was mixed with a pattern that co uld not be identified with any known phase of any hexahalide. Although the evidence is not absolutely conclusive, we believe that a new phas e was formed. If this is true, it is an open question whether the phas e 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 ph ase of transition-metal hexafluorides. Cell constants of the recogniza ble 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 ind istinguishable from those of the bulk. Variations among the shapes of orthorhombic cells of the various hexafluorides were analyzed and foun d to be related to the temperature, the bond polarity, and the bond le ngth.