Neptunium(IV) oxide or neptunium dioxide is a chemical compound with the chemical formula , 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, ), 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 (). 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 ) and a hyperstoichiometric (which has more oxygen than in the chemical formula, represented as but more accurately ) phase. Stoichiometric neptunium(IV) oxide (exactly ) shows a fluorite structure, and has and ions. Compared to this, the hypostoichiometric phase shows oxygen vacancies and existence of ions, and the hyperstoichiometric phase 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 () and bis(dimethoxyethane)neptunium tetrachloride (), 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 From oxalate
The oxalate route is the main method of production, originating in the 1960s. In it, neptunium(IV) oxalate, , is prepared through a two-stage precipitation method. Production of the oxalate starts with a nitric acid () solution, which can be prepared via ion exchange. Either hydrazine () or hydrazinium nitrate () is added to stabilize neptunium's +4 oxidation state, and either ascorbic acid () or iron(II) sulfamate () 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, ), which is then dried in air.
(ascorbic acid reduction)
(iron(II) sulfamate reduction)
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. During decomposition, neptunium(IV) oxalate hexahydrate first loses water between 80 and 200 °C to produce the anhydrous form (). decomposes further at higher temperatures, first mainly to neptunyl(V) oxalate at 270 °C and eventually to the oxide at even higher temperatures.
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 () 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 (), which is then heated further to 1185 °C to ensure complete conversion to neptunium(IV) oxide.
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 product. 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. One such compound is neptunyl ammonium nitrate (), prepared by the evaporation of a nitric acid solution containing neptunium and ammonium ions.
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 () 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.
Properties
Neptunium(IV) oxide containing neptunium-237 (237Np), the most stable and chemically significant form of neptunium, is radioactive due to the presence of 237Np. 237Np decays by alpha radiation into protactinium-233 (233Pa), which emits strong gamma rays.
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, which has been described by researchers as "mysterious". This phase transition was first discovered in 1953. One hypothesis explained this by stating that neptunium had a +3 oxidation state rather than +4, 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 , but muon spin spectroscopy showed that the transition was magnetic in origin. 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. The low-temperature phase is antiferromagnetic.
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. The ions present in neptunium(IV) oxide are and , and the ion has an electron configuration of 5f3 for its valence electrons.
Structure
Stoichiometric neptunium(IV) oxide (corresponding to exactly ) displays a fluorite structure, with lattice constant a=5.4334 Å. Like all fluorite structure materials, it forms cubic crystals (specifically face-centered cubic) with a space group of Fmm. Neptunium is eight-coordinate, with a coordination geometry of cubic, and oxygen is four-coordinate, with a tetrahedral coordination geometry. This same structure is adopted by all other actinide dioxides (, where An is an actinide). The lattice constant of these compounds decreases with increasing atomic number. A hyperstoichiometric form of neptunium(IV) oxide is known. Despite its representation as , similarities to plutonium(IV) oxide suggest that its actual composition is more accurately described as . 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 () or a neptunyl () group. It also shows evidence of hydroxide groups. The compound displays phase separation into and . Neptunium(IV) oxide shows a hypostoichiometric phase as well, represented , where x is at most 0.1. The departure from the fluorite-structured phase is attributed to the formation of oxygen vacancies, ions, and, at higher temperatures, oxygen Frenkel pairs.
Neptunium(IV) oxide produced from the calcination of neptunium(IV) oxalate () retains its shape on heating. It is polycrystalline (made of individual grains), and the size of these grains increases when the temperature 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. 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. Polycrystalline 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.
+ Neptunium(IV) oxide grain size based on calcination temperature of neptunium(IV) oxalate Calcination temperature(°C)400500600700800900 Average grain size (nm)11.05 ± 2.7820.18 ± 5.3544.77 ± 29.4494.14 ± 33.90213.78 ± 81.05371.80 ± 126.50
Relationship to related compounds
Neptunium(IV) oxide is one of two solid neptunium oxides, the other being neptunium(V) oxide (). An oxide of composition has been suggested, which would fill in the gap between with a fluorite structure and with a layered structure, though it has not been synthesized in bulk. is unstable, and only exists at low temperatures; it decomposes into neptunium(IV) oxide and oxygen gas between 700 and 973 K.
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. 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 is heated in a reducing atmosphere, for example, in hydrogen gas between 1400 and 2800 K; when heated in an oxidizing environment, stoichiometric remains stable. Its chemical formula phase is , and at 2300 K, x has a maximum of 0.1. Neptunium(IV) oxide (both stoichiometric and hyperstoichiometric) melts congruently at a temperature of 3070 ± 67 K, with composition between and .
As a gas, neptunium(IV) oxide is one of three gaseous neptunium oxides, the others being neptunium monoxide (NpO) and neptunium trioxide (). How much of each of these species, along with neptunium gas, is present, depends on the temperature and amount of oxygen. When neptunium(IV) oxide is vaporized, gaseous NpO and are formed.
Reactions Dissolution