Neptunyl ammonium nitrate
Chemical compound
From Wikipedia, the free encyclopedia
Neptunyl ammonium nitrate[4][2] is a chemical compound with the chemical formula NH4NpO2(NO3)3. It is prepared by dissolving neptunyl(VI) nitrate (NpO2(NO3)2·xH2O) in nitric acid (HNO3), addition of ammonium nitrate (NH4NO3), and evaporation of the solution. Its structure consists of ammonium (NH+4) cations, as well as NpO2(NO3)−3 anions, which contain a neptunyl (NpO2+2) group and three nitrate (NO−3) groups. Other, related compounds of the form MAnO2(NO3)3 (M = NH4, K, Rb, Cs, (CH3)4N; An = U, Np, Pu) are prepared similarly, many of which adopt the same structure. Neptunyl ammonium nitrate decomposes at high temperatures, producing neptunium(V) oxide, Np2O5, and eventually neptunium(IV) oxide, NpO2. It can be used to produce these neptunium oxides.
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| Identifiers | |||
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3D model (JSmol) |
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| Properties | |||
| NH4NpO2(NO3)3 | |||
| Molar mass | 473 g·mol−1 | ||
| Appearance | Brown solid[1] | ||
| Melting point | 150 °C (302 °F; 423 K)[2] (decomposes) | ||
| Structure | |||
| Trigonal[3] | |||
| R3c[3] | |||
a = 9.1700 Å, b = 9.1700 Å, c = 18.902 Å | |||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Synthesis
Neptunyl ammonium nitrate is prepared from neptunyl(VI) nitrate, NpO2(NO3)2·xH2O. One method of synthesizing this compound involves dissolution of neptunyl(V) hydroxide (NpO2OH) with dilute nitric acid (HNO3). Neptunium is then oxidized by bubbling the solution with ozone (O3), and the solution is evaporated under nitrogen to produce neptunyl(VI) nitrate. NpO2(NO3)2·xH2O can also be produced by adding concentrated nitric acid to a stock solution containing neptunium(V) or neptunium(VI). Neptunyl ammonium nitrate is produced upon dissolving neptunyl(VI) nitrate in nitric acid, adding ammonium nitrate, and letting the solution evaporate.[2][1][3][5]
Structure
3_-_complex_in_NH4NpO2(NO3)3.png)
Neptunyl ammonium nitrate adopts the same structure as many other actinide nitrate compounds with formulas MAnO2(NO3)3 (M = NH4, K, Rb, Cs; An = U, Np, Pu), with the exception of KUO2(NO3)3. It contains the NpO2(NO3)−3 ion, which has neptunium atoms bonded to three bidentate nitrate groups. Neptunium is in its +6 oxidation state, and is part of a neptunyl (NpO2+2) group. These neptunyl groups are linear, featuring a neptunium atom in the center forming triple bonds with two oxygen atoms. The bonds have a bond length of 1.748 Å. Neptunium atoms have a coordination geometry of hexagonal bipyramidal. The bond lengths between the neptunium atoms and the oxygen atoms within the nitrate groups are 2.467 Å. The nitrogen atoms within the ammonium groups lie directly between oxygen atoms in the neptunyl groups, with a O−N bond length of 2.97 Å. It is suggested that significant hydrogen bonding exists between the hydrogen atoms from ammonium and the oxygen atoms from neptunyl.[1][3][6][2]
Decomposition
Neptunyl ammonium nitrate decomposes at high temperatures in four steps. The first step, occurring at 150 °C, is exothermic. Because it releases ammonia, NH3, suggesting the loss of ammonium, it is attributed to the loss of ammonium nitrate (NH4NO3) to form a neptunyl(VI) nitrate intermediate, NpO2(NO3)2. The next two steps, which are endothermic and occur at 250 °C and 300 °C, are attributed to decomposition of neptunyl(VI) nitrate. X-ray diffraction shows the result of these decomposition steps to be neptunium(V) oxide, Np2O5. It decomposes further at 575 °C, resulting in the final product of neptunium(IV) oxide, NpO2.[2]
Uses
The thermal decomposition of neptunyl ammonium nitrate has been reported as a method for production of neptunium(IV) oxide, NpO2, which is used as a target for production of plutonium-238. The decomposition has been studied to gain a better understanding of the decomposition of other neptunium ammonium nitrate double salts for the modified direct denitration process of producing NpO2. Neptunyl ammonium nitrate can also be used to produce neptunium(V) oxide (Np2O5) through heating to 350 °C.[2][4]
Related compounds
Many other compounds with the formulas MAnO2(NO3)3 (M = NH4, K, Rb, Cs, (CH3)4N; An = U, Np, Pu) are known. They are prepared similarly to NH4NpO2(NO3)3; dissolution of the corresponding actinide nitrate, AnO2(NO3)2·xH2O, with nitric acid (HNO3), addition of MNO3, and evaporation of the resulting solution produces these compounds. Most of them adopt the same structure as neptunyl ammonium nitrate, with the exception of KUO2(NO3)3 and the (CH3)4N compounds.[3][1][5]