Plutonium sulfides

Plutonium sulfides are compounds of plutonium and sulfur, where sulfur exists as sulfide or polysulfide ions and plutonium exists in the trivalent state or tetravalent state. They have a general formula Pu_xS_y. Known plutonium sulfides include PuS, Pu₃S₄, Pu₅S₇, Pu₂S₃, and PuS₂. Plutonium oxysulfides (mixed oxide-sulfides) are also known, including Pu₂O₂S, Pu₄O₄S₃, PuOS, and Pu₂O₂S₃. Many of them are isostructural to the corresponding neptunium sulfides.

Plutonium monosulfide

Plutonium monosulfide is a yellow solid with the formula PuS. Unlike neptunium and uranium monosulfide, it contains plutonium in its tetravalent state, rather than trivalent. It is produced in several chemical reactions:

It is formed during the reaction of plutonium metal and sulfur gas:

It is also formed when plutonium sesquisulfide is reduced by plutonium hydride.

Reacting ground plutonium metal with hydrogen sulfide also produces plutonium monosulfide.

It is nonmagnetic, and is a semiconductor with a high electrical resistivity and a small energy gap.

It is a non-stoichiometric compound, having a range between around PuS_0.95 and PuS_1.00. At Pu:S ratios lower than 0.95 or higher than 1.00, it exists in equilibrium with plutonium metal or plutonium sesquisulfide, respectively.

At room temperature and pressure, it adopts the rock salt structure. Its structure is cubic, with space group Fm3m and lattice parameter a=5.23–5.24 Å depending on stoichiometry; its lattice parameter decreases with decreasing sulfur content. It does not exhibit a phase transition up to 60 GPa, but theoretical predictions it should undergo a phase transition to a caesium chloride-type structure at 105 GPa, undergoing a 3.6% volume loss.

Plutonium sesquisulfide

Plutonium sesquisulfide has the formula Pu₂S₃. It can be formed from the reaction between plutonium metal and sulfur gas:

Or from the thermal decomposition of plutonium disulfide:

Three polymorphs of plutonium sesquisulfide are known: α-Pu₂S₃, β-Pu₂S₃ and γ-Pu₂S₃, though β-Pu₂S₃ is a ternary oxysulfide and can feature incorporated oxygen.

Like with the neptunium sulfides, α-Pu₂S₃ is a stoichiometric compound, while β-Pu₂S₃ and γ-Pu₂S₃ have variable composition. β-Pu₂S₃ is a solid solution between Pu₁₀S₁₄O and Pu₂S₃ (formula Pu₁₀S_15-xO_x) and γ-Pu₂S₃ is substoichiometric with an ideal composition of Pu₃S₄.

α-Pu₂S₃ is the dominant form of Pu₂S₃ up to 1100 °C. At 1100 °C, it decomposes to the β-Pu₂S₃, and at 1550 °C, β-Pu₂S₃ decomposes to γ-Pu₂S₃. γ-Pu₂S₃ melts above 1700 °C; however, its melting point is dependent on its stoichiometry. Pu₂S₃ melts around 1725 °C, but Pu₃S₄ melts around 1820 °C.

Structural properties

α-Pu₂S₃ has the same structure as the related rare earth compounds, having the La₂S₃-type structure, isostructural with α-Np₂S₃ and α-Ce₂S₃. It features a framework of PuS₇ and PuS₈ polyhedra. Its crystals are orthorhombic, with lattice parameters a=3.97, b=7.37, and c=15.45 Å. It has a density of 8.31 g/cm³ and space group Pnma.

While early reports suggested β-Pu₂S₃ was a binary substoichiometric sulfide, it was later shown that the rare earth sesquisulfide phases, including β-Pu₂S₃, actually contain variable amounts of oxygen as opposed to sulfur vacancies, and it is now known to have variable composition between Pu₂S₃ and Pu₁₀S₁₄O, with a single site where oxygen and sulfur substitute for each other. It adopts a complex tetragonal structure of space group I4₁/acd which notably contains a Pu₄O tetrahedron. One crystal of this compound was found to have lattice parameters a=14.90, b=19.78 Å.

γ-Pu₂S₃ adopts a Th₃P₄-type structure of space group I3d and bcc symmetry, where each plutonium atoms is coordinated to 8 atoms of sulfur. It extends over a large range of stoichiometries, from Pu₃S₄ to Pu₂S₃. Its lattice parameter is dependent on its stoichiometry; Pu₃S₄ has lattice parameter around a=8.415 Å, while Pu₂S₃ has lattice parameter a=8.453–8.459 Å, though this depends on the exact conditions.

Plutonium disulfide

Plutonium disulfide has the formula PuS₂. It is the highest sulfide of plutonium. It is formed by the reaction of plutonium hydride and sulfur in a sealed tube for one week at about 350 °C to 750 °C, or by reacting plutonium and sulfur vapor together:

It adopts an anti-Fe₂As type structure, with distortions that arise from disulfide bonding between sulfur atoms. It is often substoichiometric, and its composition range extends from PuS_2.0 to around PuS_1.76. Its structure can be either tetragonal (space group P4/nmm) or monoclinic (space group P2₁/a), though the monoclinic phase can only be found at the exact composition PuS₂. Its lattice parameters vary depending on sulfur content; PuS_1.76 has lattice parameters a=3.936 and c=7.958 Å, PuS_1.9 has lattice parameters a=3.943 and c=7.962 Å, and tetragonal PuS_2.0 has lattice parameters a=3.974 and c=7.947 Å. The monoclinic PuS_2.0 phase has lattice parameters a=7.962, b=3.981, c=7.962 Å, and β=90°.

Plutonium disulfide is thermally unstable, and upon calcination, it decomposes to lower plutonium sulfides. PuS₂ first loses sulfur at 500 °C to give PuS_1.9, which further loses sulfur at 580 °C to give α-Pu₂S₃.

See also

• Neptunium sulfides
• Plutonium oxysulfides

References