Hexafluoride volatility

The reactivity of fluorine compounds varies from extremely stable, eg, compounds such as sulfur hexafluoride [2551-62 ] nitrogen trifluoride [7783-54-2] and the perfluorocarbons (see Fluorine compounds, organic) to extremely reactive, eg, the halogen fluorides. Another unique property of nonionic metal fluorides is great volatiUty. Volatile compounds such as tungsten hexafluoride [7783-82-6] molybdenum hexafluoride [7783-77-9] ... 

Fluorination of tungsten and rhenium produces tungsten hexafluoride, WF, and rhenium hexafluoride [10049-17-9J, ReF, respectively. These volatile metal fluorides are used in the chemical vapor deposition industry to produce metal coatings and intricately shaped components (see Thin films,... 

Molybdenum hexafluoride [7783-77-9] MoF, is a volatile liquid at room temperature. It is very moisture sensitive, hydrolysing immediately upon contact with water to produce HF and molybdenum oxyfluorides. MoF should therefore be handled in a closed system or in a vacuum line located in a chemical hood. The crystals possess a body-centered cubic stmcture that changes to orthorhombic below —96° C (1,2). The known physical properties are Hsted in Table 1. 

Elemental tellurium and the stable teUurides of heavy nonferrous metals are relatively inert and do not represent a significant health hazard (43—47). Other, more reactive teUurides, including soluble and volatile teUurium compounds such as hydrogen teUuride [7783-09-7] teUurium hexafluoride [7783-80-4] and alkyl teUurides, should be handled with caution. Some of these materials can enter the body by absorption through the skin or by inhalation and ingestion of dust or fumes. No serious consequences or deaths have been reported in workers exposed to teUurium and its compounds in industry (48). 

The octahedral hexafluorides are obtained directly from the elements and both are volatile, extremely reactive and corrosive solids, RhFg being the least stable of the platinum metal hexafluorides and reacting with glass even when carefully dried. They are thermally unstable and must be frozen out from the hot gaseous reaction mixtures, otherwise they dissociate. 

Our discussion concentrates on the uranium-235 isotope. It makes up only about 0.7% of naturally occurring uranium. The more abundant isotope, uranium-238, does not undergo fission. The first process used to separate these isotopes, and until recently the only one available, was that of gaseous effusion (Chapter 5). The volatile compound uranium hexafluoride, UF6, which sublimes at 56°C, is used for this purpose. 

Like several other heavy metals, osmium forms a volatile (bright yellow) hexafluoride (m.p. 33.2°C, b.p. 47°C)... 

At this time it was reported to us that the problems encountered in the use of uranium hexafluoride had been overcome, so there was no longer any need for other volatile compounds of uranium. 

Chemical and physical properties of the hexafluorides recently have been reviewed by Weinstock (321), who also accounted for the structures of these volatile compounds in the solid state. The vibrational properties of hexafluoride molecules have been summarized by Weinstock and Gooiman (325). 

The structural information we have of pentafluorides in the solid state is relatively new. The similar melting points (near 100° C and below) and even more so the almost identical boiling points (close to 230°) of the transition metal fluorides MeFs point to similar structures of these compounds. Their high volatility is clearly less than that of the hexafluorides so that one may assume associated aggregates or polymere molecules in the solid state. New structure analyses showed this assumption to be true. There exist at least three structure types within the 12 pentafluorides of d-transition elements hitherto known. Two crystal... 

There is no evidence of any transition metal tetrafluoride being tetra-hedrally coordinated in the solid state, however. Thus the tetrafluorides are much less volatile than the hexafluorides are. Euler and Westrum 99) conclude from thermodynamic data on Tip4 that this compound is intermediary between molecular CF4 and 8-coordinated Zrp4. 

Caution. Metal hexafluorides are volatile, toxic, corrosive, and highly hygroscopic materials. They must be handled in a very dry, clean, fluorine-preconditioned metal vacuum system. The vacuum system should be constructed from stainless steel or Monel materials (glass is not acceptable if pure products are desired). Hydrogen fluoride also is quite toxic and volatile. If only one metal... 

The pentafluoride products are easily recovered by vacuum evaporation of the HF, SiF4, and excess MF6. These volatile substances are passed through soda-lime and charcoal traps to scrub the HF and MF6. For each hexafluoride a... 

There have been some unsuccessful attempts to prepare a volatile hexafluoride from fluorine and polonium-210 26, 104), but recently such a fluoride has been prepared in this way from polonium-208 plated on platinum 132). The product appears to be stable while in the vapor phase, but on cooling a nonvolatile compound is formed, probably polonium tetrafluoride resulting from radiation decomposition of the hexafluoride. Analytical data are not recorded for any polonium fluoride, largely owing to the difficulty of determining fluoride ion accurately at the microgram level. 

A few substances were collected in bags. These analytes are generally very volatile and weakly sorbed, even on charcoal. They included carbon monoxide, carbon dioxide, sulfur hexafluoride, sul-furyl fluoride, methyl acetylene, and methyl acetylene/propadiene mixture. 

Pt forms a nonvolatile tetrafliioride, a pentaflnoride, and a volatile hexafluoride. The dark-red PtFj melts at 56.7"C and is very reactive. It even reacts with 02 at 21cC to form dioxygenylhexafluoropladnate(V), 02PtF6. 

Uranium hexafluoride is probably the most interesting of the uranium fluorides. Under ordinary conditions, it is a dense, white solid with a vapor pressure of about 120 hull ai room temperature. It can readily be sublimed or distilled, and it is by far the most volatile uranium compound known. Despite its high molecular weight, gaseous UFg is almost a perfect gas, and many of the properties of the vapor can be predicted from kinetic theory. 

Uranium hexafluoride, UF6, shown here, is a relatively volatile molecular compound that is used in the separation of the naturally occurring isotopes uranium-235 and uranium-236. There is only one naturally occurring isotope of fluorine, so any difference in the molar mass of the molecule is due to the uranium. 

Sulfur hexafluoride may be passed through water, dried over phosphorus pentoxide, and further purified by repeated sublimation of the solid at low temperatures and pressures. Selenium and tellurium hexafluoride are best purified by repeated sublimation at the temperature of solid carbon dioxide, care being taken to cause the middle fraction to melt after each sublimation. The less volatile impurities consist principally of lower fluorides (tetrafluorides). 

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