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// Workers AI · dad joke modeIs Neptunium(IV) oxide a good neighbor? No, it's very oxide-ious.

From Wikipedia, the free encyclopedia
(Redirected from Neptunium dioxide)
Neptunium(IV) oxide
Names
IUPAC name
Neptunium(IV) oxide
Other names
Neptunium oxide
Neptunium dioxide
Neptunia
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.031.651 Edit this at Wikidata
EC Number
  • 234-830-1
  • InChI=1S/Np.2O/q+4;2*-2
    Key: QKUTVYUEUPNRBO-UHFFFAOYSA-N
  • [O-2].[O-2].[Np+4]
Properties
NpO2
Molar mass 269 g·mol−1
Appearance Green solid[1]
Density 11.1 g/cm3[2]
Melting point 2,800 °C; 5,070 °F; 3,070 K[3]
Structure[4]: 725 
Fluorite (cubic), cF12
Fm3m, #225
a = 5.4334 Å
4
Thermochemistry[4]: 728 
66.24 ± 0.5 J·mol−1·K−1
80.3 ± 0.4 J·mol−1·K−1
-1074.0 ± 2.5 kJ·mol−1
-1021.7 ± 2.5 kJ·mol−1
Related compounds
Other anions
 Neptunium(IV) fluoride
Neptunium(IV) chloride
Neptunium disulfide
Other cations
 Thorium(IV) oxide
Protactinium(IV) oxide
Uranium(IV) oxide
Plutonium(IV) oxide
Americium(IV) oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X markN verify (what is checkYX markN ?)

Neptunium(IV) oxide or neptunium dioxide is a chemical compound with the chemical formula NpO2, composed of neptunium and oxygen. Solid neptunium(IV) oxide is one of two solid neptunium oxides, the other one being neptunium(V) oxide. It is synthesized in a variety of ways, most commonly by the chemical decomposition of other neptunium compounds (usually neptunium(IV) oxalate, Np(C2O4)2), but it is also produced by a process called modified direct denitration (MDD), which involves the calcination of a neptunium-containing aqueous solution. Environmentally, it can be formed by the hydrolysis of neptunium when it is in the +4 oxidation state, making it a relevant form of neptunium in the environment. In an oxidizing environment, it dissolves in water, being first converted into a mixed oxide-hydroxide phase which is then oxidized, releasing the neptunium as neptunyl(V) ions (NpO+2). During dissolution, parts of the neptunium(IV) oxide solid are also broken off.

Neptunium(IV) oxide shows both a hypostoichiometric (which has less oxygen than in the chemical formula, represented as NpO2−x) and a hyperstoichiometric (which has more oxygen than in the chemical formula, represented as NpO2+x but more accurately NpO2+x−y(OH)y·zH2O) phase. Stoichiometric neptunium(IV) oxide (exactly NpO2) shows a fluorite structure, and has Np4+ and O2− ions. Compared to this, the hypostoichiometric phase shows oxygen vacancies and existence of Np3+ ions, and the hyperstoichiometric phase (which is more accurately represented as NpO2+x−y(OH)y·zH2O) shows oxidation of neptunium to the +5 oxidation state and creation of oxo and hydroxide groups. At low temperature, it transforms into a complex phase whose existence is explained by the formation of magnetic octupoles.

Neptunium(IV) oxide reacts with a variety of chemical compounds to produce neptunium halides. Two of these compounds, neptunium(IV) chloride (NpCl4) and bis(dimethoxyethane)neptunium tetrachloride (NpCl4(DME)2), are used as starting materials in neptunium chemistry. Another use of neptunium(IV) oxide is for the production of plutonium-238 as a heat source for radioisotope thermoelectric generators, used for deep space exploration. Neptunium(IV) oxide can also be incorporated into nuclear fuel like mixed oxide (MOX) fuel, or be used as a stable form of neptunium in nuclear waste or storage. If it contains neptunium-237, the most stable and chemically important form of neptunium, it is radioactive by emitting alpha particles and gamma rays.

Synthesis

[edit]

From oxalate

[edit]
A photo of neptunium(IV) oxalate (Np(C2O4)2)

The oxalate route is the main method of NpO2 production, originating in the 1960s.[5] In it, neptunium(IV) oxalate, Np(C2O4)2, is prepared through a two-stage precipitation method. Production of the oxalate starts with a nitric acid (HNO3) solution, which can be prepared via ion exchange. Either hydrazine (N2H4) or hydrazinium nitrate (N2H5NO3) is added to stabilize neptunium's +4 oxidation state, and either ascorbic acid (C6H6O6) or iron(II) sulfamate (Fe(NH2SO3)2) is added to reduce any neptunium to the +4 state. This is necessary, as presence of higher oxidation states, namely the +5 oxidation state, reduces the amount of neptunium filtered out. At room temperature and low nitric acid concentrations, ascorbic acid is a slow reducing agent, so the reduction is done at either elevated temperatures (~50 °C) or high concentrations of nitric acid (>4 M). Addition of oxalic acid to the nitric acid solution precipitates neptunium(IV) oxalate (specifically the hexahydrate, Np(C2O4)2·6H2O), which is then dried in air.[6][7][8][9]

2 NpO+2 + 6 H+ + C6H8O6 → 2 Np4+ + C6H6O6 + 4 H2O (ascorbic acid reduction)[10]
NpO+2 + Fe2+ + 4 H+ → Np4+ + Fe3+ + 2 H2O (iron(II) sulfamate reduction)[11]
Np4+ + 2 H2C2O4 + 6 H2O → Np(C2O4)2·6H2O + 4 H+

Neptunium(IV) oxide is prepared from the oxalate through thermal decomposition. First, the neptunium(IV) oxalate is heated in a stream of nitrogen or air from room temperature to 150 °C over a 1 hour period. Afterwards, the temperature is increased. Heating to 500–550 °C provides neptunium(IV) oxide of satisfactory quality, but temperatures between 400 °C and 900 °C will yield pure neptunium(IV) oxide as well.[6][7][8][12] During decomposition, neptunium(IV) oxalate hexahydrate first loses water between 80 and 200 °C to produce the anhydrous form (Np(C2O4)2). Np(C2O4)2 decomposes further at higher temperatures, first mainly to neptunyl(V) oxalate at 270 °C and eventually to the oxide at even higher temperatures.[13][14]

Np(C2O4)2·6H2O → Np(C2O4)2·2H2O + 4 H2O
Np(C2O4)2·2H2O → Np(C2O4)2·H2O + H2O
Np(C2O4)2·H2O → Np(C2O4)2 + H2O
2 Np(C2O4)2 + 2 O2 → (NpO2)2C2O4 + 6 CO2
(NpO2)2C2O4 → 2 NpO2 + 2 CO2

In modified direct denitration

[edit]

Oak Ridge National Laboratory produces neptunium(IV) oxide using a process called modified direct denitration (MDD). This process starts from a solution of neptunium nitrate, where neptunium is in the +5 oxidation state. Ammonium nitrate (NH4NO3) is added such that there is a 2.5:1 ratio of ammonium to neptunium. The resulting solution is fed into a rotary kiln and heated to 675 °C. This produces a mixture of neptunium(IV) oxide and neptunium(V) oxide (Np2O5), which is then heated further to 1185 °C to ensure complete conversion to neptunium(IV) oxide.[5][15][16][17]

Other methods

[edit]

Neptunium(IV) oxide can be produced through the direct denitration (DD) process. In this process, a solution containing neptunium, purified by ion exchange, is partially dried and calcined in a furnace to produce the NpO2 product.[15] In addition, heating various different neptunium(IV), neptunium(V), or neptunium(VI) compounds, like hydroxides, nitrates, or oxalates, at 600–1000 °C causes them to decompose to produce neptunium(IV) oxide.[18] One such compound is neptunyl ammonium nitrate (NH4NpO2(NO3)3), prepared by the evaporation of a nitric acid solution containing neptunium and ammonium ions.[19][15]

A preparation method of neptunium(IV) oxide has been reported through synthesis of neptunium(IV) peroxide. After the nitric acid solution is prepared through ion exchange, and hydrazine is added, as in the oxalate route, hydrogen peroxide (H2O2) precipitates the peroxide. Hydrogen peroxide rapidly reduces neptunium to the +4 oxidation state, so there is no need to use reducing agents like ascorbic acid. This method offers less purification from impurities and is more sensitive than the oxalate method, so the oxalate method is preferred to it.[7]

Properties

[edit]

Neptunium(IV) oxide containing neptunium-237 (237Np), the most stable[3] and chemically significant form of neptunium,[4]: 700  is radioactive due to the presence of 237Np. 237Np decays by alpha radiation into protactinium-233 (233Pa), which emits strong gamma rays.[20]

Magnetic and electronic properties

[edit]

Neptunium(IV) oxide is an electrical insulator with a band gap of 2.85 eV. At elevated temperature, neptunium(IV) oxide is paramagnetic. Below 25.4 K, however, it transforms into a complex phase,[21] which has been described by researchers as "mysterious". This phase transition was first discovered in 1953.[22][23] One hypothesis explained this by stating that neptunium had a +3 oxidation state rather than +4,[24] but neptunium's +4 oxidation state was proven by Mössbauer spectroscopy. This phase transition was also suggested to be a result of a change in the crystal structure of NpO2, but muon spin spectroscopy showed that the transition was magnetic in origin.[25] The idea that the transition is mainly caused by magnetic dipoles or magnetic quadrupoles is inconsistent with observations, so it is proposed that the transition is primarily caused by magnetic octupoles. Bragg diffraction suggests that octupoles as well as hexadecapoles are the main contributors to the phase.[22][24][26] The low-temperature phase is antiferromagnetic.[21]

According to density functional theory calculations, the bonds in neptunium(IV) oxide are mainly ionic, but they do show small amounts of covalent character. This covalent character arises from orbital hybridization, mainly from the 5f electrons in neptunium and 2p electrons in oxygen, but there exist some contributions from neptunium's 6d electrons.[27] The ions present in neptunium(IV) oxide are Np4+ and O2−,[28] and the Np4+ ion has an electron configuration of 5f3 for its valence electrons.[29]

Structure

[edit]
The crystal structure of neptunium(IV) oxide. Np4+: __ O2−: __

Stoichiometric neptunium(IV) oxide (corresponding to exactly NpO2) displays a fluorite structure, with lattice constant a=5.4334 Å.[30][4]: 725  Like all fluorite structure materials, it forms cubic crystals (specifically face-centered cubic) with a space group of Fm3m.[31]: S591  Neptunium is eight-coordinate, with a coordination geometry of cubic, and oxygen is four-coordinate, with a tetrahedral coordination geometry.[32] This same structure is adopted by all other actinide dioxides (AnO2, where An is an actinide). The lattice constant of these compounds decreases with increasing atomic number.[33]: 2389  A hyperstoichiometric form of neptunium(IV) oxide is known. Despite its representation as NpO2+x, similarities to plutonium(IV) oxide suggest that its actual composition is more accurately described as NpO2+x−y(OH)y·zH2O. The value of x shows a wide range, up to 0.25. The lattice constant is not changed with increasing x. XAFS shows its structure to contain short neptunium-oxygen bonds around 1.85–1.90 Å, suggestive of an oxo (Np=O) or a neptunyl (O=Np=O) group. It also shows evidence of hydroxide groups. The compound displays phase separation into NpO2 and Np4O9.[30] Neptunium(IV) oxide shows a hypostoichiometric phase as well, represented NpO2−x, where x is at most 0.1. The departure from the fluorite-structured NpO2.0 phase is attributed to the formation of oxygen vacancies, Np3+ ions, and, at higher temperatures, oxygen Frenkel pairs.[34][35]

Neptunium(IV) oxide produced from the calcination of neptunium(IV) oxalate (Np(C2O4)2) retains its shape on heating. It is polycrystalline (made of individual grains), and the size of these grains increases when the temperature Np(C2O4)2 is heated to is higher. The surface area and size of the grain boundaries decreases upon heating, and the particle size is between 11 and 371 nm.[12] When produced from the dilution of a neptunium carbonate solution, neptunium(IV) is also a polycrystalline material, in this case showing a grain size of 2–5 nm.[36] Polycrystalline NpO2 is additionally produced through the direct denitration (DD) and modified direct denitration (MDD) methods. In DD, the grain size is approximately 100 nm, whereas in MDD, the grain size is approximately 500–1000 nm.[15]

Neptunium(IV) oxide grain size based on calcination temperature of neptunium(IV) oxalate[12]
Calcination temperature(°C) 400 500 600 700 800 900
Average grain size (nm) 11.05 ± 2.78 20.18 ± 5.35 44.77 ± 29.44 94.14 ± 33.90 213.78 ± 81.05 371.80 ± 126.50
[edit]

Neptunium(IV) oxide is one of two solid neptunium oxides, the other being neptunium(V) oxide (Np2O5). An oxide of composition Np4O9 has been suggested, which would fill in the gap between NpO2 with a fluorite structure and Np2O5 with a layered structure, though it has not been synthesized in bulk. Np2O5 is unstable, and only exists at low temperatures; it decomposes into neptunium(IV) oxide and oxygen gas between 700 and 973 K.[20][34]

2 Np2O5 → 4 NpO2 + O2

The hyperstoichiometric phase of neptunium(IV) oxide is formed by the oxidation and hydrolysis of stoichiometric neptunium(IV) oxide, reactions which also occur for the corresponding plutonium compound plutonium(IV) oxide.[30] Neptunium(IV) oxide shows a hypostoichiometric phase at temperatures above 1300 K, which decomposes into stoichiometric neptunium(IV) oxide and metallic neptunium at lower temperatures. This phase is formed when stoichiometric NpO2 is heated in a reducing atmosphere, for example, in hydrogen gas between 1400 and 2800 K; when heated in an oxidizing environment, stoichiometric NpO2 remains stable.[34][37] Its chemical formula phase is NpO2−x, and at 2300 K, x has a maximum of 0.1.[35] Neptunium(IV) oxide (both stoichiometric and hyperstoichiometric) melts congruently at a temperature of 3070 ± 67 K, with composition between NpO1.98 and NpO2.[3]

As a gas, neptunium(IV) oxide is one of three gaseous neptunium oxides, the others being neptunium monoxide (NpO) and neptunium trioxide (NpO3). How much of each of these species, along with neptunium gas, is present, depends on the temperature and amount of oxygen.[35] When neptunium(IV) oxide is vaporized, gaseous NpO and NpO2 are formed.[34]

Reactions

[edit]

Dissolution

[edit]

In reducing conditions, neptunium(IV) oxide is highly insoluble.[38] However, under oxidizing environments, neptunium(IV) oxide is partially dissolved in water. While neptunium in its +4 oxidation state as in NpO2 does not dissolve easily, upon oxidation it can dissolve with conversion to higher oxidation states.[18] Dissolution of NpO2 under these conditions can produce colloidal and ionic neptunium species, and primarily occurs at grain boundaries (boundaries between particles in a solid). Dissolution occurs quickly at first (within 4 weeks), but afterwards becomes more gradual. During dissolution, sections of neptunium(IV) oxide can break off, which is what likely forms the colloids. The production of ionic neptunium species, NpO+2, happens first through the reaction of neptunium(IV) oxide at the grain boundaries to produce a neptunium hydroxide phase, NpOx(OH)y·zH2O. This phase is then oxidized, releasing NpO+2 into solution.[38] How fast neptunium(IV) oxide dissolves strongly depends on the size of the individual particles that make up the solid; when they are larger, the surface area and grain boundaries are smaller, so the dissolution occurs much slower than when the particles are smaller.[12]

Neptunium(IV) oxide dissolves in nitric acid (HNO3). When refluxing at concentrations of 0.8 to 4.0 M HNO3, the dissolution occurs takes between 8 and 10 hours, and the dissolution time is independent of HNO3 concentration. Refluxing at higher concentrations (e.g. 12 M) is used to dissolve neptunium(IV) oxide as well.[39][40] It also dissolves in a mixture of hydrofluoric acid and concentrated hydrochloric acid.[41] It is slightly dissolved by aqueous solutions containing sodium carbonate (Na2CO3) and hydrogen peroxide (H2O2). Oxidizing agents like ammonium persulfate ((NH4)2S2O8) do not significantly increase the amount dissolved, which does not exceed 1.5–1.6%. Sonication can increase the amount dissolved to 25%.[42]

Halogenation

[edit]

At high temperatures, neptunium fluorides can form from the reaction of neptunium(IV) oxide with hydrogen fluoride (HF). With hydrogen gas (H2) present, neptunium(III) fluoride (NpF3) is formed at 500 °C, but if hydrogen is not added, neptunium(IV) fluoride (NpF4) is formed. NpF4 is also formed when neptunium(IV) oxide is treated with a gaseous mixture of oxygen (O2) and HF at 450–600 °C or when NpO2 is heated between 280 and 330 °C in fluorine (F2). Heating NpO2 in fluorine at 500 °C produces neptunium hexafluoride (NpF6), with neptunyl fluoride (NpO2F2) as an intermediate. Neptunium hexafluoride is also formed when neptunium(IV) oxide reacts with dioxygen difluoride (O2F2).[4]: 730–732 [43]: 101–108 

2 NpO2 + H2 + 6 HF → 2 NpF3 + 4 H2O
NpO2 + 4 HF → NpF4 + 2 H2O
NpO2 + x O2F2 → NpF6 + (x + 1) O2 + (x - 3) F2

Neptunium(IV) oxide reacts with carbon tetrachloride (CCl4) between 280 and 500 °C to produce neptunium(IV) chloride. At 350–400 °C, reaction of neptunium(IV) oxide with CCl4 as well as H2 reduces it, producing neptunium(III) chloride (NpCl3). Reaction of hydrogen bromide (HBr) or hydrogen iodide (HI) with neptunium(IV) oxide at 500 °C provides neptunium(III) bromide (NpBr3) and neptunium(III) iodide (NpI3), respectively. Neptunium(IV) bromide (NpBr4) is prepared by the reaction of neptunium(IV) oxide with aluminium(III) bromide (AlBr3). If aluminium metal is added as well, NpBr3 is formed instead. A reaction of aluminium(III) iodide, AlI3, with neptunium(IV) oxide forms NpI3.[4]: 736–738 [43]: 129, 135, 148, 154 

3 NpO2 + 4 AlBr3 → 3 NpBr4 + 2 Al2O3

Uses

[edit]

In plutonium production

[edit]
Glowing graphite cube containing plutonium-238 oxide
The 238PuO2 radioisotope thermoelectric generator of the Curiosity rover

Neptunium(IV) oxide is used for the production of plutonium-238 for radioisotope thermoelectric generators (RTGs), such as those used by the National Aeronautics and Space Administration (NASA) for deep space exploration.[5][44] After preparation, it is treated to remove impurities like protactinium-233 to reduce radiation. Then, it is mixed with powdered aluminium, and the resulting mixture is pressed into pellets. The pellets are then irradiated to convert neptunium-237 to plutonium-238. The conversion happens through neutron capture.[5][6] After irradiation, the pellets are dissolved, first in a nitrate (NO−3) solution to remove the aluminium, and then in nitric acid (HNO3) to dissolve the neptunium and plutonium. Solvent extraction is used to isolate the plutonium and the neptunium, separating them from fission products. The plutonium and neptunium are purified by ion exchange. The plutonium is converted to the final product of plutonium(IV) oxide (PuO2), which is used as the heat source for the RTGs. The neptunium is converted back into neptunium(IV) oxide and used to make more plutonium.[44]

In storage and waste

[edit]

As a stable neptunium compound, neptunium(IV) oxide is used as a form of neptunium for storage. For example, neptunium-237 for the production of plutonium-238 is stored as NpO2, such as at Idaho National Laboratory.[15][45][46] It is also a form of neptunium found in legacy nuclear waste.[47][48]

Mixed oxide fuel

[edit]

Neptunium(IV) oxide can be used in mixed oxide fuel (MOX fuel) for nuclear reactors like fast-neutron reactors. When MOX fuel is used up, the reprocessing of the resulting spent nuclear fuel isolates neptunium and other minor actinides, which are formed from radioactive decay and neutron capture, and can be reused and fed back into the fuel. Using neptunium(IV) oxide in fuel converts neptunium-237 to isotopes that are lighter and less radioactive, reducing its toxicity. In MOX fuel, neptunium(IV) oxide is incorporated with other actinide oxides, like thorium(IV) oxide, uranium(IV) oxide, and plutonium(IV) oxide. There are two ways to incorporate NpO2 and other minor actinide oxides into fuel: homogeneous mode, where the oxides are incorporated into the main fuel at 2–6% concentration, and heterogeneous, where they are separated from the main fuel and incorporated at 10–30% concentration.[49][50][51][52]: 149 

As a precursor

[edit]

Neptunium(IV) oxide is used as a precursor to other neptunium compounds. For example, it can be used in the synthesis of a common stating material in neptunium chemistry, bis(dimethoxyethane)neptunium tetrachloride (NpCl4(DME)2; DME = dimethoxyethane, CH3OCH2CH2OCH3). Dissolution of neptunium(IV) oxide in hydrofluoric acid/concentrated hydrochloric acid mixture and subsequent drying produces a residue. Dissolving this residue in dimethoxyethane, addition of trimethylsilyl chloride (Si(CH3)3Cl), drying, and washing with diethyl ether gives pink crystals of NpCl4(DME)2 as the product.[41]

Another important starting material prepared from NpO2 is neptunium(IV) chloride, NpCl4, through the reaction with carbon tetrachloride (CCl4) at high temperatures (>600 °C) and sublimation at higher temperatures to purify the product. This compound in turn is used to create other starting materials, caesium hexachloroneptunate(IV) (Cs2NpCl6) and tetraethylammonium hexachloroneptunate(IV) ([(C2H5)4N]2NpCl6), through reaction with caesium chloride (CsCl) and tetraethylammonium chloride ([(C2H5)4N]2Cl), respectively, in hydrochloric acid. However, these methods are hindered by the hazardous conditions needed to prepare NpCl4.[53][54]

In the environment

[edit]

In aqueous solution, actinides from nuclear waste in their +4 oxidation state often undergo hydrolysis, forming actinide oxide or oxide-hydroxide particles and colloids, which, in the case of neptunium, can can eventually form neptunium(IV) oxide. Along with neptunium(IV) oxide's stability, this makes it a relevant form of neptunium in the environment. Dilution of a mildly basic neptunium carbonate solution containing Np(CO3)6−5 ions converts the ions through olation to neptunium oxide colloids, which precipiate as nanocrystalline neptunium(IV) oxide. This method of formation is expected to occur in geological environments.[36]

Neptunium(IV) oxide is highly insoluble in reducing conditions, which limits neptunium's ability to travel in the environment. Under oxidizing conditions, however, neptunium(IV) oxide can potentially be dissolved and oxidized to form the neptunyl(V) cation, NpO+2. This cation is extremely soluble and mobile. Dissolution can also cause parts of neptunium(IV) oxide to break off and form colloids.[38]

References

[edit]
  1. ↑ Patnaik, Pradyot (2003). Handbook of Inorganic Chemicals (PDF). McGraw-Hill. p. 605. ISBN 978-0-07-049439-8.
  2. ↑ www.webelements.com: Neptunium dioxide.
  3. 1 2 3 Böhler, R.; Welland, M. J.; Bruycker, F. De; Boboridis, K.; Janssen, A.; Eloirdi, R.; Konings, R. J. M.; Manara, D. (2012). "Revisiting the melting temperature of NpO2 and the challenges associated with high temperature actinide compound measurements". Journal of Applied Physics. 111 (11) 113501. doi:10.1063/1.4721655.
  4. 1 2 3 4 5 6 Yoshida, Zenko; Johnson, Stephen G.; Kimura, Takaumi; Krsul, John R. (2010). "Neptunium". The Chemistry of the Actinide and Transactinide Elements (PDF). pp. 699–812. doi:10.1007/978-94-007-0211-0_6. ISBN 978-94-007-0210-3.
  5. 1 2 3 4 Peruski, Kathryn M.; Parker, Connor J.; Cary, Samantha K. (December 2023). "Analysis of neptunium oxides produced through modified direct denitration". Journal of Nuclear Materials. 587 154704. Bibcode:2023JNuM..58754704P. doi:10.1016/j.jnucmat.2023.154704. OSTI 2000387.
  6. 1 2 3 Luerkens, D. W. (1983). Two-stage precipitation of neptunium (IV) oxalate (Report). doi:10.2172/5860637. OSTI 5860637.
  7. 1 2 3 Porter, J. A. (1964). "Production of Neptunium Dioxide". Industrial & Engineering Chemistry Process Design and Development. 3 (4): 289–292. doi:10.1021/i260012a001.
  8. 1 2 Porter, J.; Dukes, E. (1961). Precipitation of Neptunium Oxalate and Calcination to Neptunium Oxide (Report). doi:10.2172/1838350. OSTI 1838350.
  9. ↑ Jordan, J.; Watkins, R.; Hensel, S. (July 2009). Neptunium Oxide Processing. 50th Annual Meeting of the Institute of Nuclear Materials Management. OSTI 958296.
  10. ↑ Kim, Eung-Ho; Chung, Dong-Yong; Kim, Won-Ho; Shin, Young-Joon; Lee, Eil-Hee; Yoo, Jae-Hyung; Choi, Cheong-Song (1997). "Neptunium Oxalate Precipitation from the Simulated Radioactive Liquid Waste". Journal of Nuclear Science and Technology. 34 (3): 283–287. doi:10.3327/jnst.34.283.
  11. ↑ Baranov, A. A.; Erin, E. A.; Nagaitseva, L. V.; Chistyakov, V. M. (2014). "Kinetics of neptunium(V) reduction with iron(II) sulfamate in nitric acid solutions". Radiochemistry. 56 (1): 6–10. Bibcode:2014Radch..56....6B. doi:10.1134/S1066362214010020.
  12. 1 2 3 4 Peruski, Kathryn M.; Powell, Brian A. (2020). "Effect of calcination temperature on neptunium dioxide microstructure and dissolution". Environmental Science: Nano. 7 (12): 3869–3876. doi:10.1039/d0en00689k. OSTI 1756008.
  13. ↑ Kozlova, R. D.; Karelin, A. I.; Lobas, O. P.; Matyukha, V. A. (1984). "[Thermal decomposition of neptunium (4) oxalate]". Радиохимия [Radiokhimiya] (in Russian). 26 (3): 311–316. hdl:2027/uc1.b3535039. ISSN 0033-8311.
  14. ↑ Karelin, A. I.; Krot, N. N.; Kozlova, R. D.; Lobas, O. P.; Matukha, V. A. (1990). "Thermal decomposition of Np(IV) and Pu(III, IV) oxalates". Journal of Radioanalytical and Nuclear Chemistry Articles. 143 (1): 241–252. Bibcode:1990JRNC..143..241K. doi:10.1007/BF02117565.
  15. 1 2 3 4 5 Gilson, Sara E.; Lawson, Kathryn M.; Dyke, Thomas; Rogers, Kayron; Keever, Tamara; Miskowiec, Andrew; Spano, Tyler L. (March 2026). "Understanding the structural and morphological effects of synthesis route on NpO2". Journal of Nuclear Materials. 623 156460. doi:10.1016/j.jnucmat.2026.156460. OSTI 3017002.
  16. ↑ Felker, L. K.; Vedder, R. J.; Walker, E. A. (November 2006). Preparation of Mixed Oxides by Modified Direct Denitration. Transactions of the American Nuclear Society. Vol. 95. pp. 195–196.
  17. ↑ Vedder, Raymond (2018). "Oak Ridge National Laboratory Preparation of Sintered 237NpO2 Pellets for Irradiation to Produce 238Pu Oxide". doi:10.2172/1484146.
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  19. ↑ Lawson, Kathryn M.; Spano, Tyler L.; Roach, Jordan M.; Parker, Connor J.; Isbill, Sara B.; Miskowiec, Andrew (2025). "Thermal decomposition of neptunyl ammonium nitrate: Mechanistic insights and structural characterization of the Np2O5 intermediate phase". Inorganic Chemistry Frontiers. 12 (19): 5915–5925. doi:10.1039/d5qi01015b.
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  21. 1 2 Pegg, James T.; Shields, Ashley E.; Storr, Mark T.; Wills, Andrew S.; Scanlon, David O.; De Leeuw, Nora H. (2019). "Magnetic structure of UO2 and NpO2 by first-principle methods" (PDF). Physical Chemistry Chemical Physics. 21 (2): 760–771. doi:10.1039/c8cp03581d. PMID 30547167.
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  23. ↑ Hotta, Takashi (2009). "Microscopic analysis of multipole susceptibility of actinide dioxides: A scenario of multipole ordering in AmO2". Physical Review B. 80 (2) 024408. arXiv:0906.3607. Bibcode:2009PhRvB..80b4408H. doi:10.1103/PhysRevB.80.024408.
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  25. ↑ Kopmann, W.; Litterst, F.J; Klauß, H.-H; Hillberg, M.; Wagener, W.; Kalvius, G.M; Schreier, E.; Burghart, F.J; Rebizant, J.; Lander, G.H (1998). "Magnetic order in NpO2 and UO2 studied by muon spin rotation" (PDF). Journal of Alloys and Compounds. 271–273: 463–466. doi:10.1016/S0925-8388(98)00112-1.
  26. ↑ Lovesey, S. W.; Balcar, E.; Detlefs, C.; Laan, G van der; Sivia, D. S.; Staub, U. (2003). "Neptunium octupole and hexadecapole motifs in NpO2directly from electric dipole (E1) enhanced x-ray Bragg diffraction". Journal of Physics: Condensed Matter. 15 (26): 4511–4518. doi:10.1088/0953-8984/15/26/301.
  27. ↑ Li, Shi-Chang; Guo, Yong; Gao, Tao; Ao, Bing-Yun (2015). "The structural, electronic, and optical properties of NpO2 and PuO2: A hybrid density-functional-theory study". The European Physical Journal B. 88 (9) 230. doi:10.1140/epjb/e2015-50681-9.
  28. ↑ Pegg, James T.; Aparicio-Anglès, Xavier; Storr, Mark; De Leeuw, Nora H. (2017). "DFT+U study of the structures and properties of the actinide dioxides". Journal of Nuclear Materials. 492: 269–278. doi:10.1016/j.jnucmat.2017.05.025.
  29. ↑ Veal, B.W.; Lam, D.J.; Diamond, H. (1977). "X-ray photoelectron spectroscopy of 5f electrons in dioxides of neptunium and plutonium". Physica B+C. 86–88: 1193–1194. doi:10.1016/0378-4363(77)90841-5. OSTI 7124452.
  30. 1 2 3 Tayal, Akhil; Conradson, Steven D.; Baldinozzi, Gianguido; Namdeo, Sonu; Roberts, Kevin E.; Allen, Patrick G.; Shuh, David K. (July 2017). "Identification of NpO2+x in the binary Np-O system". Journal of Nuclear Materials. 490: 279–287. doi:10.1016/j.jnucmat.2017.04.019. OSTI 1510745.
  31. ↑ Mehl, Michael J.; Hicks, David; Toher, Cormac; Levy, Ohad; Hanson, Robert M.; Hart, Gus; Curtarolo, Stefano (2017). "The AFLOW Library of Crystallographic Prototypes: Part 1". Computational Materials Science. 136: S1–S828. doi:10.1016/j.commatsci.2017.01.017.
  32. ↑ Wyckoff, Ralph W.G. (1948). Crystal Structures. pp. Chapter IV, text page 2. Retrieved 22 Apr 2025.
  33. ↑ Gutowski, Keith E.; Bridges, Nicholas J.; Rogers, Robin D. (2010). "Actinide Structural Chemistry". The Chemistry of the Actinide and Transactinide Elements. pp. 2380–2523. doi:10.1007/978-94-007-0211-0_22. ISBN 978-94-007-0210-3.
  34. 1 2 3 4 Smith, A.L.; Colle, J.-Y.; Beneš, O.; Konings, R.J.M.; Sundman, B.; Guéneau, C. (December 2016). "Thermodynamic assessment of the neptunium–oxygen system: Mass spectrometric studies and thermodynamic modelling". The Journal of Chemical Thermodynamics. 103: 257–275. Bibcode:2016JChTh.103..257S. doi:10.1016/j.jct.2016.07.040.
  35. 1 2 3 Guéneau, Christine; Chartier, Alain; Fossati, Paul; Van Brutzel, Laurent; Martin, Philippe (2020). "Thermodynamic and Thermophysical Properties of the Actinide Oxides". Comprehensive Nuclear Materials. pp. 111–154. doi:10.1016/B978-0-12-803581-8.11786-2. ISBN 978-0-08-102866-7.
  36. 1 2 Husar, Richard; Hübner, René; Hennig, Christoph; Martin, Philippe M.; Chollet, Mélanie; Weiss, Stephan; Stumpf, Thorsten; Zänker, Harald; Ikeda-Ohno, Atsushi (2015). "Intrinsic formation of nanocrystalline neptunium dioxide under neutral aqueous conditions relevant to deep geological repositories". Chemical Communications. 51 (7): 1301–1304. doi:10.1039/C4CC08103J. PMID 25479067.
  37. ↑ Richter, K.; Sari, C. (1987). "Phase relationships in the neptunium-oxygen system". Journal of Nuclear Materials. 148 (3): 266–271. doi:10.1016/0022-3115(87)90019-5.
  38. 1 2 3 Peruski, Kathryn M.; Koehler, Kelliann C.; Powell, Brian A. (2020). "Grain boundary facilitated dissolution of nanocrystalline NpO
    2    (s) from legacy waste processing". Environmental Science: Nano. 7 (8): 2293–2301. doi:10.1039/D0EN00262C. OSTI 1637555.
  39. ↑ Rudisill, T S (7 June 2002). Dissolution of Neptunium Oxide in Unirradiated Mark 53 Targets (Report). doi:10.2172/799453. OSTI 799453.
  40. ↑ Xu, N.; Tandon, L.; Gallimore, D.; Lujan, E.; Porterfield, D. R.; Colletti, L.; Kuhn, K. (April 2013). "Dissolution and assay of neptunium oxide". Journal of Radioanalytical and Nuclear Chemistry. 296 (1): 245–249. Bibcode:2013JRNC..296..245X. doi:10.1007/S10967-012-1947-Z.
  41. 1 2 Whitefoot, Megan A.; Perales, Diana; Zeller, Matthias; Bart, Suzanne C. (23 December 2021). "Synthesis of Non-Aqueous Neptunium(III) Halide Solvates from NpO 2". Chemistry – A European Journal. 27 (72): 18054–18057. doi:10.1002/chem.202103265. OSTI 1976299. PMID 34643978.
  42. ↑ Chervyakov, N.M.; Boyarintsev, A.V.; Perevalov, S.A.; Stepanov, S.I.; Vinokurov, S.E. (2021). "Oxidative Dissolution of Neptunium(IV) Oxide in Carbonate Solutions" (PDF). RAD Conference Proceedings. doi:10.21175/RadProc.2021.18.
  43. 1 2 Becker, Ramona; Hartwig, Helga; Köppe, Herbert; Vanecek, Hans; Velić, Paul; Warncke, Rudolf; Zelle, Anna (1978). Warncke, Rudolf (ed.). Gmelin Handbuch der Anorganischen Chemie. doi:10.1007/978-3-662-06224-1. ISBN 978-3-662-06226-5.
  44. 1 2 DePaoli, David; Benker, Dennis; Delmau, Lætitia; Sherman, Steven; Riley, Frank; Bailey, Porter; Collins, Emory; Wham, Robert. Process Development for Plutonium-238 Production at Oak Ridge National Laboratory. Nuclear and Emerging Technologies for Space 2019. Richland, Washington, United States of America.
  45. ↑ "Supplement Analysis for the Programmatic Environmental Impact Statement (PEIS) for Accomplishing Expanded Civilian Nuclear Energy Research and Development and Isotope Production Missions in the United States, Including the Role of the Fast Flux Test Facility". United States Department of Energy. Retrieved Jun 9, 2026.
  46. ↑ Wham, Robert; DePaoli, David; Collins, Emory D.; Delmau, Laetitia; Benker, Dennis (September 2019). Process Chemistry for the Production of Plutonium-238. GLOBAL 2019. Seattle, Washington, United States of America. OSTI 1766431.
  47. ↑ "Keeping waste where it belongs: Grain size explains how spent nuclear fuel enters the environment" (Press release). United States Department of Energy. 16 September 2021.
  48. ↑ Pool, Rebecca (21 September 2021). "How grain size keeps nuclear waste where it belongs". Wiley Analytical Science.
  49. ↑ Ghosh, P.S.; Arya, A.K. "Thermo-physical and Diffusional Properties of Uranium-Neptunium and Thorium-Neptunium Mixed Oxides" (PDF). Bhabha Atomic Research Center Newsletter. Retrieved 9 June 2026.
  50. ↑ Chollet, Mélanie; Prieur, Damien; Böhler, Robert; Belin, Renaud; Manara, Dario (October 2015). "The melting behaviour of uranium/neptunium mixed oxides". The Journal of Chemical Thermodynamics. 89: 27–34. Bibcode:2015JChTh..89...27C. doi:10.1016/j.jct.2015.04.031.
  51. ↑ Eser, E.; Koc, H.; Gokbulut, M.; Gursoy, G. (December 2014). "Estimations of heat capacities for actinide dioxide: UO2, NpO2, ThO2, and PuO2". Nuclear Engineering and Technology. 46 (6): 863–868. doi:10.5516/NET.07.2014.024. hdl:20.500.12639/1397.
  52. ↑ "Actinide and Fission Product Partitioning and Transmutation". May 1999. Retrieved 9 June 2026.
  53. ↑ Arnold, Polly L.; Dutkiewicz, Michał S.; Walter, Olaf (13 September 2017). "Organometallic Neptunium Chemistry". Chemical Reviews. 117 (17): 11460–11475. doi:10.1021/acs.chemrev.7b00192. hdl:20.500.11820/ced916d2-1cff-4bb9-bb37-aa32a52f9dc1. PMID 28853564.
  54. ↑ Johnson, Aaron T.; Parker, T. Gannon; Dickens, Sara M.; Pfeiffer, Jana K.; Oliver, Allen G.; Wall, Donald; Wall, Nathalie A.; Finck, Martha R.; Carney, Kevin P. (6 November 2017). "Synthesis and Crystal Structures of Volatile Neptunium(IV) β-Diketonates". Inorganic Chemistry. 56 (21): 13553–13561. doi:10.1021/acs.inorgchem.7b02290. OSTI 1478397. PMID 29039922.
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