Neptunyl carbonates are compounds containingneptunyl (NpO+2 orNpO2+2) andcarbonate (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 ofneptunium innuclear waste.[2]
While there are reports of a binary neptunyl(V)bicarbonate,NpO2HCO3, these results are dubious.[3]
Many ternary neptunyl(V) carbonates are known. One example is the compoundNH4NpO2CO3, which is precipitated from neptunium(VI) solutions byammonium carbonate. It consists ofammonium ions sandwiched between anionic sheets made up ofNpO+2 andCO2−3 ions. In these sheets, each neptunyl group is coordinated to three carbonate groups, and each carbonate group is coordinated to three neptunyl groups. Isostructural compoundsKNpO2CO3,RbNpO2CO3, andCsNpO2CO3, containingpotassium,rubidium, andcaesium ions respectively, are also known. These compounds havespace groupP63/mmc and have ahexagonal structure.NH4NpO2CO3 has lattice parametersa=5.09858 andc=10.89394 Å, whileKNpO2CO3 has lattice parametersa=5.0994 andc=10.2210 Å.[1][2]
Ternary neptunyl(V) carbonates containingsodium are also known. The anhydrous form ofNaNpO2CO3 has neptunyl carbonate sheets with the same structure asuranyl carbonate akarutherfordine. Each neptunyl group is bonded to four carbonate groups, with two of them beingbidentate and two of them beingmonodentate.[4] It is precipitated from aqueous solutions as follows:[3]
Hydrated forms are also known, such asNaNpO2CO3·3H2O andNaNpO2CO3·3.5H2O.[1] They precipitate from aqueous solution as follows:[3]
The compoundNaNpO2CO3·3.5H2O has a layered structure in thetriclinic crystal system, with space groupP1 and lattice parametersa=4.3420,b=4.8962,c=10.0933 Å,α=91.014°,β=77.834°, andγ=90.004°.[1]
WhenNaNpO2CO3 reacts with high concentrations ofsodium carbonate, it forms the more stable solidNa3NpO2(CO3)2.[5][3]
A lithium double salt,LiNpO2CO3·2H2O, has also been characterized. It consists of layers of composition[LiNpO2CO3H2O]n, along withwaters of crystallization. In each layer, neptunium is bonded to three bidentate carbonate groups.[6][4]
Inaqueous solution with carbonate, neptunyl(VI) can bond to up to three carbonate ligands as inNpO2(CO3)4−3. At neptunium concentrations higher than 1mM, it converts to the trinuclear species(NpO2)3(CO3)6−6, as governed by the equation:[5]
Binary neptunyl(VI) carbonate,NpO2CO3, can be formed by addinglithium carbonate to an acidic solution containingneptunyl(VI) nitrate. It adopts the same structure asuranyl carbonate, and has lattice parametersa=4.82,b=9.17, andc=4.24 Å.[7]
Neptunyl(VI) forms several ternary carbonates, many of which involve theNpO2(CO3)4−3 ion. These include theguanidinium salt(C(NH2)3)4NpO2(CO3)3, thetetramethylammonium salt,(N(CH3)4)4NpO2(CO3)3·6H2O, and the caesium salt,Cs4NpO2(CO3)3·8H2O. In theNpO2(CO3)4−3 ion, the neptunyl group is surrounded by three bidentate carbonateligands.[4] Compounds involving sodium (Na4NpO2(CO3)3), potassium (K4NpO2(CO3)3), and ammonium ((NH4)4NpO2(CO3)3) are also known, but as of 1998 their structures have not been characterized.[5][needs update]