Neptunium(IV) oxide, or neptunium dioxide, is a

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

, olive green cubic crystalline solid with the formula NpO₂. It is one of two stable

oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...

s of

neptunium Neptunium is a chemical element; it has chemical symbol, symbol Np and atomic number 93. A radioactivity, radioactive actinide metal, neptunium is the first transuranic element. It is named after Neptune, the planet beyond Uranus in the Solar Syste ...

, the other being neptunium(V) oxide. It emits both α- and γ-particles.

Production

Industrially, neptunium dioxide is formed by

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

of neptunium(IV) oxalate, followed by

calcination Calcination is thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), generally f ...

to neptunium dioxide. Production starts with a

nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...

feed solution containing neptunium ions in various oxidation states. First, a

hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydraz ...

inhibitor is added to slow any oxidation from standing in air. Then

ascorbic acid Ascorbic acid is an organic compound with formula , originally called hexuronic acid. It is a white solid, but impure samples can appear yellowish. It dissolves freely in water to give mildly acidic solutions. It is a mild reducing agent. Asco ...

reduces the feed solution to predominantly neptunium(IV): :2Np⁵⁺ + C₆H₈O₆ → 2Np⁴⁺ + C₆H₆O₆ + 2H⁺ :Np⁶⁺ + C₆H₈O₆ → Np⁴⁺ + C₆H₆O₆ + 2H⁺ Addition of

oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and chemical formula , also written as or or . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name i ...

precipitates hydrated neptunium oxalate... :Np⁴⁺ + 2H₂C₂O₄ + 6H₂O → Np(C₂O₄)₂.6H₂O(v) + 4H⁺ ...which pyrolyzes when heated: Equations extrapolated from verbal description. :Np(C₂O₄)₂.6H₂O Np(C₂O₄)₂ NpO₂ + 2CO(g) Neptunium dioxide can also be formed from precipitation of neptunium(IV) peroxide, but the process is much more sensitive.

Purification

As a byproduct of nuclear fission reactors, neptunium dioxide can be purified by

fluorination In chemistry, halogenation is a chemical reaction which introduces one or more halogens into a chemical compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, drugs. ...

, followed by reduction with excess calcium in the presence of iodine. However, the aforementioned synthesis yields a quite pure solid, with less than 0.3% mass fraction of impurities. Generally, further purification is unnecessary.

Structure

Due to neptunium's large size, neptunium dioxide has a

fluorite structure The fluorite structure refers to a common motif for compounds with the formula MX2. The X ions occupy the eight tetrahedral interstitial sites whereas M ions occupy the regular sites of a face-centered cubic In crystallography, the cubic (or is ...

, with lattice constant ''a''=5.43 Å. Like all fluorite structure materials, it has a

space group In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...

of Fmm. Neptunium is eight-

coordinate In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine and standardize the position of the points or other geometric elements on a manifold such as Euclidean space. The coordinates are ...

, with a

cubic Cubic may refer to: Science and mathematics * Cube (algebra), "cubic" measurement * Cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex ** Cubic crystal system, a crystal system w ...

coordination geometry The coordination geometry of an atom is the geometrical pattern defined by the atoms around the central atom. The term is commonly applied in the field of inorganic chemistry, where diverse structures are observed. The coordination geometry depen ...

, and oxygen is four-coordinate, with a

tetrahedral In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...

coordinate geometry.

Other properties

Neptunium dioxide contributes to the α-decay of ²⁴¹Am, reducing its usual

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

by an untested but appreciable amount. The compound has a low

specific heat capacity In thermodynamics, the specific heat capacity (symbol ) of a substance is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. It is also referred to as massic heat ...

(900 K, compared with

uranium dioxide Uranium dioxide or uranium(IV) oxide (), also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reac ...

's specific heat capacity of 1400 K), an abnormality theorized to stem from its 5f electron count. Another unique trait of neptunium dioxide is its "mysterious low-temperature ordered phase". Mentioned above, it references an abnormal level of order for an actinide dioxide complex at low temperature. Further discussion of such topics could indicate useful physical trends in the actinides.

Uses

The neptunium dioxide complex is used as a means of stabilizing, and decreasing the "long term environmental burden" of neptunium as a nuclear fission byproduct. Actinide-containing spent nuclear fuel will commonly be treated so that various AnO₂ (where An = U, Np, Pu, etc.) complexes form. In neptunium dioxide, neptunium is of reduced radiotoxicity compared with elemental neptunium and is thus more desirable for storage and disposal. Neptunium dioxide is also used experimentally for research into nuclear chemistry and physics, and it is speculated that it could be used to make efficient nuclear weapons. In nuclear reactors, neptunium dioxide can also be used as the target for plutonium bombardment. Furthermore, a patent for a rocket powered by neptunium dioxide is held by Shirakawa Toshihisa, but there is little information available into research and production associated with such a product.

References

{{DEFAULTSORT:Neptunium(Iv) Oxide Neptunium compounds Oxides Fluorite crystal structure