Plutonium(III) chloride

Plutonium(III) chloride
Names

IUPAC name Plutonium(III) chloride
Other names Plutonium trichloride
Identifiers
CAS Number	13569-62-5^Y[PubChem]
3D model (JSmol)	Interactive image
ChemSpider	14483818^Y
PubChem CID	19902959
CompTox Dashboard (EPA)	DTXSID90601161
InChI InChI=1S/3ClH.Pu/h31H;/q;;;+3/p-3^Y Key: CYMMZQWRMUVJRR-UHFFFAOYSA-K^Y InChI=1S/3ClH.Pu/h31H;/q;;;+3/p-3 Key: CYMMZQWRMUVJRR-DFZHHIFOAR Key: CYMMZQWRMUVJRR-UHFFFAOYSA-K
SMILES [Pu+3].[Cl-].[Cl-].[Cl-]
Properties
Chemical formula	Cl₃Pu
Molar mass	350.322 g/mol
Appearance	Green solid
Density	5.71 g/cm³, solid^[1]
Melting point	767 °C (1,413 °F; 1,040 K)^[1]
Boiling point	1,767 °C (3,213 °F; 2,040 K)^[1]
Related compounds
Other anions	Plutonium(III) fluoride Plutonium(III) bromide Plutonium(III) iodide
Other cations	Samarium(III) chloride
Related plutonium chlorides	Plutonium(IV) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is ^YN ?) Infobox references

Plutonium(III) chloride or plutonium trichloride is a chemical compound with the formula Pu Cl₃. 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 (containing no water), or in the form of hydrates (solids containing water), such as the hexahydrate, PuCl₃·6H₂O. It is used in the processing of plutonium metal and in molten-salt reactors.

Plutonium(III) chloride can be synthesized through a variety of reactions, many involving the reaction of other plutonium compounds, such as plutonium(III) oxalate (Pu₂(C₂O₄)₃), plutonium(IV) oxide (PuO₂), or plutonium hydride (PuH_x), with chlorinating agents like hydrogen chloride (HCl), hexachloropropene (C₃Cl₆), phosgene (COCl₂), carbon tetrachloride (CCl₄), or chlorine gas (Cl₂). The structure of both anhydrous plutonium(III) chloride and PuCl₃·6H₂O are known.

Synthesis

Multiple different methods have been used to synthesize plutonium(III) chloride, which all involve the chlorination of plutonium or plutonium compounds. Many of these methods use plutonium(III) oxalate, which can be prepared by carefully adding oxalic acid to an acidic plutonium(III) solution, resulting in the decahydrate, Pu₂(C₂O₄)₃·10H₂O.^[2]^{: 1092–1093}

After it is synthesized, it is advised that it be purified by sublimation in a sealed quartz tube.^[2]^{: 1092–1093}

Using hydrogen chloride

For medium-scale reactions (between 1 and 10 grams), the best method of preparing plutonium(III) chloride is by reacting plutonium(III) oxalate with hydrogen chloride, HCl. The reaction proceeds like so:^[2]^: 1093

Pu₂(C₂O₄)₃·10H₂O + 6 HCl → 2 PuCl₃ + 3 CO₂ + 3 CO + 13 H₂O

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 PuCl₃ is melted at 800 °C and sparged with HCl.^[2]^: 1093

Plutonium(III) chloride can also be formed from aqueous solutions of hydrogen chloride (hydrochloric acid). Upon evaporation of the HCl solution, a residue of plutonium(III) chloride hexahydrate, PuCl₃·6H₂O, is formed. The hexahydrate can then be dehydrated in a stream of HCl to give the anhydrous solid.^[2]^: 1093

Hydrogen chloride also corrodes plutonium metal:^[3]

2 Pu + 6 HCl → 2 PuCl₃ + 3 H₂

Using hexachloropropene

Due to the difficulty in handling hazardous gases, hexachloropropene (C₃Cl₆) can be used as a chlorinating agent instead of more dangerous compounds. Plutonium(III) oxalate (Pu₂(C₂O₄)₃) has been reported to be an efficient initial source of plutonium. To produce plutonium(III) chloride, Pu₂(C₂O₄)₃ is heated with C₃Cl₆:^[4]

Pu₂(C₂O₄)₃·10H₂O + 3 C₃Cl₆ → 2 PuCl₃ + 3 C₃Cl₄O + 3 CO₂ + 3 CO + 10 H₂O

Using chlorine gas

Plutonium(III) 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.^[2]^: 1093 Chlorine can also be used to chlorinate plutonium(IV) oxide (PuO₂). For PuO₂ chlorination, carbon is used as a reducing agent. The reaction proceeds as follows:^[5]

2 PuO₂ + 3 C + 3 Cl₂ → 2 PuCl₃ + 2 CO + CO₂

Using other gaseous chlorinating agents

Plutonium(IV) oxide can also be chlorinated with phosgene (COCl₂) or carbon tetrachloride, yielding analytically pure samples of PuCl₃. This method has been used at Los Alamos National Laboratory. One method for the continuous production of PuCl₃ using phosgene is as follows:^[2]^: 1093

Plutonium(IV) oxalate hydrate (Pu(C₂O₄)₂·6H₂O) is precipitated from plutonium(IV) solution, dried, and calcined (heated to decomposition without oxygen) to form plutonium(IV) oxide (PuO₂). The resulting PuO₂ is then heated with phosgene in a tube furnace at 500 °C. This method produces plutonium at a rate of 250 grams / hour.^[2]^: 1093

Using ammonium chloride

To ensure a high-purity product, plutonium(III) 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 PuCl₃. The overall reaction follows like so:^[6]

2 Pu + 6 NH₄Cl → 2 PuCl₃ + 6 NH₃ + 3 H₂

Properties

The color of plutonium(III) chloride depends on its method of production. Plutonium(III) 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.^[2]^: 1093 It is incredibly hygroscopic, and is readily hydrated by atmospheric moisture.^[5] This leads to the formation of several hydrates, or compounds containing water. These include the monohydrate (PuCl₃·H₂O), dihydrate (PuCl₃·2H₂O), and hexahydrate (PuCl₃·6H₂O). Which hydrate is formed depends on the partial pressure of water in the atmosphere.^[2]^: 1100 Plutonium(III) 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.^[7]^{: 130, 140}

It is the only stable solid binary chloride of plutonium, but other chlorides are known in the gas phase. When heated with chlorine at high temperatures, plutonium(III) chloride reacts to form the compound plutonium tetrachloride, increasing its volatility. Upon condensing, it reverts to PuCl₃.^[2]^{: 1092, 1094}

2 PuCl₃ + Cl₂ ⇌ 2 PuCl₄

The compound is antiferromagnetic below around 4.5 K.^[8]

Structure

Anhydrous PuCl₃ is isostructural with (has the same structure as) the related uranium(III) chloride, UCl₃, 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(III) bromide (PuBr₃) and plutonium(III) iodide (PuI₃); 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.^[2]^{: 1096–1097} Unlike in PuBr₃, where the Pu-Br bonds are predominantly covalent, the Pu-Cl bonds within PuCl₃ are predominantly ionic.^[2]^: 1100

The crystal structure of PuCl₃. Blue is plutonium, green is chlorine.

The coordination sphere of each plutonium atom in the structure of PuCl₃. Blue is plutonium, green is chlorine.

The coordination sphere of each chlorine atom in the structure of PuCl₃. Blue is plutonium, green is chlorine.

Plutonium(III) chloride hexahydrate, PuCl₃·6H₂O, adopts the GdCl₃·6H₂O-type structure,^{[note 1]} 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 PuCl₂(H₂O)+6 groups. The PuCl₂(H₂O)+6 groups are linked together by chloride ions via hydrogen bonding.^[10]

Plutonium(III) chloride trihydrate (PuCl₃·3H₂O) adopts the same structure as the corresponding neodymium compound, NdCl₃·3H₂O.^[5]

Uses

Plutonium metal processing

Plutonium(III) chloride is an important intermediate in the production of plutonium metal. Plutonium metal produced from the reduction of plutonium tetrafluoride and plutonium(IV) 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(III) 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:^[2]^{: 869–870}

2 Am + 3 MgCl₂ → 2 AmCl₃ + 3 Mg

2 Pu + 3 MgCl₂ → 2 PuCl₃ + 3 Mg

Am + PuCl₃ → AmCl₃ + Pu

After the americium is oxidized, the americium(III) chloride and molten salt mixture can be separated, leaving a plutonium metal button behind.^[2]^: 870

Molten-salt reactors

Plutonium(III) chloride is a common nuclear material for use in molten-salt reactors, reactors which use molten salts with actinides dissolved in them as their fuel, such as a mixture of sodium chloride and plutonium(III) chloride. Molten salts containing chlorides, like plutonium(III) chloride, are often used, as they have low melting points and can dissolve high concentrations of actinides.^[4]^[5]^[6]

Handling and storage

Due to plutonium(III) chloride's high hygroscopicity, it must be kept in an controlled, dehumidified atmosphere.^[5]

Notes

PuCl₃·6H₂O is known to be isostructural with SmCl₃·6H₂O,^[9]^: 357 which adopts the GdCl₃·6H₂O-type structure.^[10]

References

www.webelements.com: Plutonium(III) chloride.
Clark, David L.; Hecker, Siegfried S.; Jarvinen, Gordon D.; Neu, Mary P. (2011). "Plutonium". The Chemistry of the Actinide and Transactinide Elements (PDF). doi:10.1007/978-94-007-0211-0_7. ISBN 978-94-007-0211-0.
Burgess, M.; Haschke, J. M.; Allen, T. H.; Morales, L. A.; Jarboe, D. M.; Puglisi, C. V. (April 1998). "Chloride-catalyzed corrosion of plutonium in glovebox atmospheres".
"Plan for synthesis of actinide chlorides and fluorides without using gaseous agents" (PDF). inis.iaea.org.
"Synthesis of actinide chlorides for molten salt preparation". airdrive.eventsair.com.
Karlsson, Toni Y.; Middlemas, Scott C.; Nguyen, Manh-Thuong; Woods, Michael E.; Tolman, Kevin R.; Glezakou, Vassiliki-Alexandra; Herrmann, Steven D.; Schorne-Pinto, Juliano; Johnson, Ryan D.; Reddish, Shawn E.; Warmann, Stephen A.; Paviet, Patricia D. (2023). "Synthesis and thermophysical property determination of NaCl-PuCl3 salts". Journal of Molecular Liquids. 387 122636. doi:10.1016/j.molliq.2023.122636.
Brown, David (1972). "Compounds with Chlorine". Transurane. pp. 129–147. doi:10.1007/978-3-662-11547-3_7. ISBN 978-3-662-11548-0.
Jones, E. R.; Hendricks, M. E.; Stone, J. A.; Karraker, D. G. (1974). "Magnetic properties of the trichlorides, tribromides, and triiodides of U(III), Np(III), and Pu(III)". The Journal of Chemical Physics. 60 (5): 2088–2094. Bibcode:1974JChPh..60.2088J. doi:10.1063/1.1681318.
Lemire, R. J. et al., Chemical Thermodynamics of Neptunium and Plutonium, Elsevier, Amsterdam, 2001.
Kandabadage, Thimira; Legnon, Beau; Baranets, Sviatoslav (2024). "Comprehensive structural study of lanthanide(III) chloride hydrates: [RECl₃·xH₂O (RE = La–Nd, Sm–Lu; x = 6, 7)]". Acta Crystallographica Section E. 80 (12): 1342–1349. Bibcode:2024AcCrE..80.1342K. doi:10.1107/S2056989024011319. PMC 11789172. PMID 39906789.