Hexafluoride melting point

In plastic crystals all or a part of the molecules rotate about their centers of gravity. Typically, plastic crystals are formed by nearly spherical molecules, for example hexafluorides like SF6 or MoF6 or white phosphorus in a temperature range immediately below the melting point. Such crystals often are soft and can be easily deformed. 

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... 

Sulphur hexafluoride is a colourless, odourless and incombustible gas of density 5-08 (air — 1). When solidified it forms a colourless crystalline mass of melting-point —56° C. As the vapour pressure of the solid attains one atmosphere at —62° C. the solid has no melting-point... 

Selenium Fluorides.—Two iluorides of selenium have been described, the hexafluoride, SeF6, and the tetrafluoride, SeF4. The former was obtained as a stable gas by the action of fluorine on selenium at 78° C.5 Its boiling-point, melting-point and critical temperature are —84-5° C. (in a sealed tube), —39° C. and +72° C., respectively. The vapour density, 97-28, agrees with the formula SeF8.6 The gas does not attack glass. 

Sulfur hexafluoride sublimes at -64 °C to produce a dense gas (6.14 g L-1). Under a pressure of 2 atm, the melting point is -51 °C. The molecule has the expected octahedral structure and a dipole moment of zero. The compound is so inert that it is used as a gaseous insulator, and rats allowed to breathe a mixture of SF6 and oxygen show no ill effects after several hours of exposure. This inertness is a result of the molecule having no vacant bonding site or unshared electron pairs on sulfur to initiate a reaction and the fact that six fluorine atoms shield the sulfur atom from attack. Consequently, there is no low-energy pathway for reactions to occur, and the compound is inert even though many reactions are thermodynamically favored. 

Tellurium hexafluoride (TeF ) is a colorless gas ivith a repulsive odor. Its melting point is — 37.6 °C and it undergoes sublimation at — 38.9°C. Tellurium hexafluoride... 

Electrowinning of Aluminum. Aluminum, the most abundant metal in the Earth s crust, did not become readily available commercially until the development of the Hall-Heroult process. This process involves electrolysis of dry aluminum oxide (alumina) dissolved in cryolite (sodium aluminum hexafluoride). Additional calcium fluoride is used to lower the melting point of the cryolite. The process runs at about 960 degrees Celsius and uses carbon electrodes. The alumina for the Hall-Heroult process is obtained from an ore called bauxite, an impure aluminum oxide with varying amounts of compounds such as iron oxide and silica. The preparation of pure alumina follows the Bayer process The alumina is extracted from the bauxite as a solution in sodium hydroxide (caustic soda), reprecipitated by acidification, filtered, and dried. The electrolysis cell has a carbon coating at the bottom that forms a cathode. 

The MXg series represents a set of molecular solids and liquids at ambient conditions, owing to the octahedral coordination of the transition metals. Many of the other second and third row metal hexafluorides show the same phenomenon. Mode and WCle have d ions in hep with 1/6 of the octahedral holes occupied. The fluorides are volatile solids with low melting and boiling points, while WCle and WBre melt around 570 K. 

Uranium hexafluoride (UF ), also called hex, is probably the best known and most widely investigated compound of uranium mainly because it is the only uranium compound with significant vapor pressure at ambient temperatures and therefore an essential raw material for most commercial isotope enrichment processes. UFg is a white monoclinic crystalline solid that sublimes directly to a gas (reaches atmospheric pressure at 56.5°C), but when heated in a closed vessel will melt at 64.05°C, which is the triple point where the solid, liquid, and gas phases coexist, as shown in Figure 1.8. This is probably one of the most weU-recognized phase diagrams in the chemical literature. 

Note that aluminum trifluoride, silicon tetrafluoride. and sulfur hexafluoride have the interesting property, as does carbon dioxide, of subliming at I atm pressure without melting. The temperatures given in the table as the boiling points of these two substances are in fact the subliming points, when the vapor pressure of the crystals becomes equal to 1 atm. 

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