Neptunyl carbonates

Neptunyl carbonates are compounds containing neptunyl (NpO+
2 or NpO2+
2) and carbonate (CO2−
3) ions. Frequently, these compounds will also include other cations.^[1] Due to the abundance of carbonate in natural water, they are relevant for disposal of neptunium in nuclear waste.^[2]

While there are reports of a binary neptunyl(V) bicarbonate, NpO
2HCO
3, these results are dubious.^[3]

Many ternary neptunyl(V) carbonates are known. One example is the compound NH
4NpO
2CO
3, which is precipitated from neptunium(VI) solutions by ammonium carbonate. It consists of ammonium ions sandwiched between anionic sheets made up of NpO+
2 and CO2−
3 ions. In these sheets, each neptunyl group is coordinated to three carbonate groups, and each carbonate group is coordinated to three neptunyl groups. Isostructural compounds KNpO
2CO
3, RbNpO
2CO
3, and CsNpO
2CO
3, containing potassium, rubidium, and caesium ions respectively, are also known. These compounds have space group P6₃/mmc and have a hexagonal structure. NH
4NpO
2CO
3 has lattice parameters a=5.09858 and c=10.89394 Å, while KNpO
2CO
3 has lattice parameters a=5.0994 and c=10.2210 Å.^[1][2]

Ternary neptunyl(V) carbonates containing sodium are also known. The anhydrous form of NaNpO
2CO
3 has neptunyl carbonate sheets with the same structure as uranyl carbonate aka rutherfordine. Each neptunyl group is bonded to four carbonate groups, with two of them being bidentate and two of them being monodentate.^[4] It is precipitated from aqueous solutions as follows:^[3]

CO2−
3 + Na+
+ NpO+
2 → NaNpO
2CO
3

Hydrated forms are also known, such as NaNpO
2CO
3 · 3H₂O and NaNpO
2CO
3 · 3.5H₂O.^[1] They precipitate from aqueous solution as follows:^[3]

CO2−
3 + Na+
+ NpO+
2 + x H
2O → NaNpO
2CO
3 · xH
2O

The compound NaNpO
2CO
3 · 3.5H₂O has a layered structure in the triclinic crystal system, with space group P1 and lattice parameters a=4.3420, b=4.8962, c=10.0933 Å, α=91.014°, β=77.834°, and γ=90.004°.^[1]

When NaNpO
2CO
3 reacts with high concentrations of sodium carbonate, it forms the more stable solid Na
3NpO
2(CO
3)
2.^[5][3]

A lithium double salt, LiNpO
2CO
3 · 2H₂O, has also been characterized. It consists of layers of composition [LiNpO
2CO
3H
2O]
n, along with waters of crystallization. In each layer, neptunium is bonded to three bidentate carbonate groups.^[6][4]

In aqueous solution with carbonate, neptunyl(VI) can bond to up to three carbonate ligands as in NpO
2(CO
3)4−
3. At neptunium concentrations higher than 1mM, it converts to the trinuclear species (NpO
2)
3(CO
3)6−
6, as governed by the equation:^[5]

3 NpO
2(CO
3)4−
3 → (NpO
2)
3(CO
3)6−
6 + 3 CO2−
3

Binary neptunyl(VI) carbonate, NpO
2CO
3, can be formed by adding lithium carbonate to an acidic solution containing neptunyl(VI) nitrate. It adopts the same structure as uranyl carbonate, and has lattice parameters a=4.82, b=9.17, and c=4.24 Å.^[7]

Neptunyl(VI) forms several ternary carbonates, many of which involve the NpO
2(CO
3)4−
3 ion. These include the guanidinium salt (C(NH
2)
3)
4NpO
2(CO
3)
3, the tetramethylammonium salt, (N(CH
3)
4)
4NpO
2(CO
3)
3 · 6H₂O, and the caesium salt, Cs
4NpO
2(CO
3)
3 · 8H₂O. In the NpO
2(CO
3)4−
3 ion, the neptunyl group is surrounded by three bidentate carbonate ligands.^[4] Compounds involving sodium (Na
4NpO
2(CO
3)
3), potassium (K
4NpO
2(CO
3)
3), and ammonium ((NH
4)
4NpO
2(CO
3)
3) are also known, but as of 1998 their structures have not been characterized.^{[5][needs update]}

• Vitorge, Pierre; Capdevila, Hélène. "Np(V) and Np(VI) in bicarbonate/carbonate aqueous solutions"

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