Plutonium(III) chloride
Plutonium chloride or plutonium trichloride is a chemical compound with the formula. It is the only stable solid chloride of plutonium, though another plutonium chloride, plutonium tetrachloride, is known in the gas phase. It can either be found as an anhydrous solid, or in the form of hydrates, such as the hexahydrate,. It is used in the processing of plutonium metal and in molten-salt reactors.
Synthesis
Multiple different methods have been used to synthesize plutonium chloride, which all involve the chlorination of plutonium or plutonium compounds. Many of these methods use plutonium oxalate, which can be prepared by carefully adding oxalic acid to an acidic plutonium solution, resulting in the decahydrate,.After it is synthesized, it is advised that it be purified by sublimation in a sealed quartz tube.
Using hydrogen chloride
For medium-scale reactions, the best method of preparing plutonium chloride is by reacting plutonium oxalate with hydrogen chloride, HCl. The reaction proceeds like so:For processing 100 gram quantities of plutonium, plutonium hydride can be reacted with hydrogen chloride in a fluidized bed reactor at 450 °C. To remove oxychlorides, the resulting is melted at 800 °C and sparged with HCl.
Plutonium chloride can also be formed from aqueous solutions of hydrogen chloride. Upon evaporation of the HCl solution, a residue of plutonium chloride hexahydrate,, is formed. The hexahydrate can then be dehydrated in a stream of HCl to give the anhydrous solid.
Using hexachloropropene
Due to the difficulty in handling hazardous gases, hexachloropropene can be used as a chlorinating agent instead of more dangerous compounds. Plutonium oxalate has been reported to be an efficient initial source of plutonium. To produce plutonium chloride, is heated with :Using chlorine gas
Plutonium chloride can be produced via reacting plutonium metal with chlorine gas at temperatures between 300 °C and 500 °C, followed by sublimation at 600–800 °C. Chlorine can also be used to chlorinate plutonium oxide. For chlorination, carbon is used as a reducing agent. The reaction proceeds as follows:Using other gaseous fluorinating agents
Plutonium oxide can also be chlorinated with phosgene or carbon tetrachloride, yielding analytically pure samples of. This method has been used at Los Alamos National Laboratory. One method for the continuous production of using phosgene is as follows:Plutonium oxalate hydrate is precipitated from plutonium solution, dried, and calcined to form plutonium oxide. The resulting is then heated with phosgene in a tube furnace at 500 °C. This method produces plutonium at a rate of 250 grams / hour.
Using ammonium chloride
To ensure a high-purity product, plutonium chloride can be prepared by reacting plutonium metal with ammonium chloride at high temperature. The ammonium chloride sublimes, producing ammonia and hydrogen chloride. The hydrogen chloride subsequently reacts with the plutonium metal to form. The overall reaction follows like so:Formation
Plutonium chloride is formed unintentionally from the corrosion of plutonium metal by hydrogen chloride gas:Properties
The color of plutonium chloride depends on its method of production. Plutonium chloride produced via dehydration of the hexahydrate is slate-blue, while when prepared by one of the anhydrous methods the compound is blue-green to emerald green. When condensated from the gas phase, it appears as an emerald green solid. It is incredibly hygroscopic, and is readily hydrated by atmospheric moisture. This leads to the formation of several hydrates, or compounds containing water. These include the monohydrate, dihydrate, and hexahydrate. Which hydrate is formed depends on the partial pressure of water in the atmosphere. Plutonium chloride hexahydrate has a melting point of 94 °C, at which it melts in its own waters of crystallization, forming a solution. In a vacuum at 27 °C, it decomposes to give the monohydrate, and upon heating between 400 and 520 °C, it gives plutonium oxychloride, PuOCl.When heated with chlorine at high temperatures, plutonium chloride reacts to form the compound plutonium tetrachloride, increasing its volatility. Upon condensing, it reverts to.
The compound is antiferromagnetic below around 4.5 K.
Structure
is isostructural with the related uranium chloride,, having a hexagonal structure. Each plutonium atom is surrounded by nine chlorine atoms. Six of the nine chlorine atoms form a triangular prism, which have Pu-Cl bond lengths of 2.886 Å. Each of three rectangular sides of the triangular prism are capped off by chlorine atoms, which have Pu-Cl distances of 2.919 Å. This gives each plutonium atom a coordination geometry of tricapped trigonal prismatic. The prisms' triangular bases connect to form infinite chains. This structure is related to the structures of plutonium bromide and plutonium iodide ; however, one of the three bromine or iodine atoms that would have capped off one of the rectangular sides of the triangular prism is too far away from the plutonium atom to bond. Unlike in, where the Pu-Br bonds are predominantly covalent, the Pu-Cl bonds within are predominantly ionic.Plutonium chloride hexahydrate,, adopts the -type structure, which is seen across all the lanthanide chlorides except those of lanthanum and cerium. In this structure, each plutonium atom is bonded to six water ligands and two chloride ligands, forming monomeric groups. The groups are linked together by chloride ions via hydrogen bonding.
Plutonium chloride trihydrate adopts the same structure as the corresponding neodymium compound,.
Uses
Plutonium metal processing
Plutonium chloride is an important intermediate in the production of plutonium metal. Plutonium metal produced from the reduction of plutonium tetrafluoride and plutonium oxide can be impure, notably with americium created via the beta decay of plutonium-241. To separate americium impurities from plutonium, the impure plutonium metal is placed in a mixture of molten magnesium chloride, sodium chloride, and potassium chloride. Plutonium chloride is produced when plutonium metal is oxidized by the magnesium chloride, but is subsequently reduced back to plutonium metal by americium. The reactions that occur are:After the americium is oxidized, the americium chloride and molten salt mixture can be separated, leaving a plutonium metal button behind.