Neptunium sulfides

Neptunium sulfides are compounds of neptunium and sulfur. In these compounds, neptunium has an oxidation state of +3 or +4, and sulfur exists as sulfide or polysulfide ions. They have the general formula Np_xS_y. Known neptunium sulfides include NpS, Np₃S₄, Np₂S₃, Np₃S₅, NpS₂, Np₂S₅, and NpS₃. These compounds are often isostructural with their corresponding uranium or plutonium compounds. Neptunium oxysulfides (mixed oxide-sulfides) are also known, including Np₂O₂S, Np₄O₄S₃, and NpOS.^[1][2][3]

Neptunium(III) sulfides

Neptunium monosulfide

Structure of neptunium monosulfide.

Neptunium monosulfide has the formula NpS. It features neptunium in the +3 state, being an electride salt like plutonium monosulfide. Like plutonium monosulfide, it adopts a rock salt structure, with lattice constant a=5.532 Å.^[3][4]

It can be produced by reducing Np₂S₃ with neptunium metal at 1600 °C:^[1]

Np₂S₃ + Np → 3 NpS

It can also be produced by reacting neptunium metal with sulfur gas:^[1]

Np + S → NpS

It behaves as a resistor, with an electrical resistivity of 60 μΩ⋅cm.^[4]

It is predicted to undergo a phase transition to a caesium chloride-type structure at 75 GPa, with a 3.7% volume loss from the NaCl-type structure.^[5]

Neptunium sesquisulfide

Structure of α-Np₂S₃.

Neptunium sesquisulfide has the formula Np₂S₃. It has several polymorphs, which are isostructural with the corresponding plutonium sulfides. It is often hypostoichiometric, with compositions ranging between Np₃S₄, Np₅S₇, and Np₂S₃.^[1][2][4]

It was first prepared by reacting neptunium dioxide with carbon disulfide and hydrogen sulfide in 1948. It can also be prepared via thermal composition of Np₃S₅ at ~1200 K in vacuum:^[1][4]

2 Np₃S₅ → 3 Np₂S₃ + S

Np₂S₃ was initially reported to be isostructural with uranium sesquisulfide (dubbed η-Np₂S₃) with lattice parameters a=10.3, b=10.6, and c=3.9 Å. However, later experiments could not reproduce these results.^[1][2][4]

Later experiments^[2][4] found evidence for three polymorphs of Np₂S₃: α-, β-, and γ-Np₂S₃. These are isostructural with the corresponding plutonium compounds.^[1]

α-Np₂S₃ is the form of Np₂S₃ present at standard temperature. It has an α-Ce₂S₃-type structure. Unlike β-Np₂S₃ and γ-Np₂S₃, it is stoichiometric. It is orthorhombic with lattice parameters a=3.98, b=7.39, and c=15.50 Å.^[1]

β-Np₂S₃ can be formed from heating α-Np₂S₃ to around ~1500 K. It has a β-Ce₂S₃-type structure, being tetragonal with lattice parameters a=14.94, b=7.39, and c=19.84 Å. It has a stoichiometry between Np₅S₇ and Np₂S_3-ε.^[4]

γ-Np₂S₃ can be formed from heating β-Np₂S₃ to around ~1800 K. It adopts the cubic Th₃P₄-type structure, being cubic with lattice parameter a=8.440 Å. It has a stoichiometry between Np₃S₄ and Np₂S_3-ε.^[1][2][4]

Neptunium(III,IV) sulfides

Trineptunium pentasulfide

Structure of trineptunium pentasulfide.

Trineptunium pentasulfide has the formula Np₃S₅. Like the corresponding neptunium selenide, Np₃Se₅, it is antiferromagnetic, undergoing magnetic ordering at 35 K. It is a black solid which is isostructural with triuranium pentasulfide.^[3]

It can be obtained by the thermal decomposition of neptunium trisulfide at 500 °C or by reacting neptunium and sulfur in caesium chloride flux.^[1][3][4]

It decomposes into α-Np₂S₃ at 900 °C. At normal temperatures, however, it is highly stable, and is a common byproduct of reactions involving compounds of neptunium and sulfur.^[3][4]

It contains Np^III and Np^IV ions in a 2:1 ratio, and its formula can be represented as (Np³⁺)₂(Np⁴⁺)(S²⁻)₅.^[3]

Neptunium(IV) sulfides

Neptunium disulfide

Neptunium disulfide has the formula NpS₂. Very little information about it is available, and it is difficult to synthesize. Mössbauer spectroscopy indicates that it contains Np^IV.^[4][6]

Dineptunium pentasulfide

Structure of dineptunium pentasulfide.

Dineptunium pentasulfide has the formula Np₂S₅. It is isostructural with the corresponding thorium sulfide (Th₂S₅) and uranium sulfide (U₂S₅). It is a polysulfide, and its formula can be represented as (Np⁴⁺)₂(S²⁻)₃(S2−2). It forms tetragonal crystals, with lattice parameters a=10.48 and c=9.84 Å. It can be synthesized by reacting Np₃S₅ with sulfur at 500 °C: ^[1][4]

2 Np₃S₅ + 5 S → 3 Np₂S₅

Neptunium trisulfide

Neptunium trisulfide has the formula NpS₃. It has been found to undergo magnetic ordering at low temperatures (~45 K). It is a polysulfide, and its formula can be represented as (Np⁴⁺)(S²⁻)(S2−2).^[1][2]

It can be formed via the reaction of neptunium and sulfur at 500 °C:

Np + 3 S → NpS₃

It has a monoclinic structure, isostructural with US₃, with lattice parameters a=5.36, b=3.87, c=18.10 Å, and β=99°.^[4]

Neptunium oxysulfides

Np₂O₂S

Dineptunium dioxide monosulfide (Np₂O₂S) is the only stable oxysulfide of neptunium at high temperatures (~1600 °C). It is isostructural with other lanthanide and actinide oxysulfides, like La₂O₂S, having a hexagonal crystal structure with lattice parameters a=3.95 and c=6.80 Å. It is formed by the high temperature decomposition of Np₄O₄S₃. It contains Np^III.^[1][4]

Np₄O₄S₃

Tetraneptunium tetroxide trisulfide (Np₄O₄S₃) is formed from the decomposition of NpOS in vacuum at 700 °C. It is isostructural with Pu₄O₄S₃, having a pseudo-hexagonal structure with lattice parameters a=4.07, b=6.76, c=3.89 Å, and β=118°. It contains Np^III and Np^IV in equal amounts. Its formula can be written as (Np³⁺)₂(Np⁴⁺)₂(O²⁻)₄(S²⁻)₃.^[4]

NpOS

Neptunium monoxide monosulfide (NpOS) is commonly encountered as a result of oxidation of other neptunium sulfides, e.g. Np₃S₅. Pure NpOS can be formed by oxidizing NpS in a sealed ampoule at 700 °C:^[3][4]

2 NpS + O₂ → 2 NpOS

It is isostructural with the corresponding uranium oxysulfide and plutonium oxysulfide, forming tetragonal crystals with lattice parameters a=3.815 and c=6.623 Å. It contains Np^IV.^[4]