Concept
Plutonium hexafluoride is the highest fluoride of plutonium, and is of interest for laser enrichment of plutonium, in particular for the production of pure plutonium-239 from irradiated uranium. This pure plutonium is needed to avoid premature ignition of low-mass nuclear weapon designs by neutrons produced by spontaneous fission of plutonium-240. Plutonium hexafluoride is prepared by fluorination of plutonium tetrafluoride (PuF4) by powerful fluorinating agents such as elemental fluorine. PuF4 + F2 → PuF6 This reaction is endothermic. The product forms relatively quickly at temperatures of 750 °C, and high yields may be obtained by quickly condensing the product and removing it from equilibrium. It can also be obtained by fluorination of plutonium(III) fluoride, plutonium(IV) oxide, or plutonium(IV) oxalate at approximately 700 °C: 2 PuF3 + 3 F2 → 2 PuF6 PuO2 + 3 F2 → PuF6 + O2 3 F2 → PuF6 + 4 CO2 Alternatively, plutonium(IV) fluoride oxidizes in an 800-°C oxygen atmosphere to plutonium hexafluoride and plutonium(IV) oxide: 3 PuF4 + O2 → 2 PuF6 + PuO2 In 1984, the synthesis of plutonium hexafluoride at near–room-temperatures was achieved through the use of dioxygen difluoride. Hydrogen fluoride is not sufficient even though it is a powerful fluorinating agent. Room temperature syntheses are also possible by using krypton difluoride or irradiation with UV light. Plutonium hexafluoride is a red-brown volatile solid, crystallizing in the orthorhombic crystal system with space group Pnma and lattice parameters a = 995 pm, b = 902 pm, and c = 526 pm. It sublimes around 60 °C with heat 12.1 kcal/mol to a gas of octahedral molecules with plutonium-fluorine bond lengths of 197.1 pm. At high pressure, the gas condenses, with a triple point at 51.58 °C and ; the heat of vaporization is 7.4 kcal/mol. At temperatures below -180 °C, plutonium hexafluoride is colorless. Plutonium hexafluoride is paramagnetic, with molar magnetic susceptibility 0.173 mm3/mol. Plutonium hexafluoride admits six different oscillation modes: stretching modes v1, v2, and v3 and rotational modes v4, v5, and v6.
