Neptunium(IV) oxide

From oxalate

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

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

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]

Magnetic and electronic properties

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

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 NpO_2+x, similarities to plutonium(IV) oxide suggest that its actual composition is more accurately described as NpO_2+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 NpO_2−x, where x is at most 0.1. The departure from the fluorite-structured NpO_2.0 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]

Relationship to related compounds

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 NpO_2−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 NpO_1.98 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]

Dissolution

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, NpO_x(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

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

In plutonium production

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

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

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

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]