Neptunium(IV) oxide

Neptunium(IV) oxide, or neptunium dioxide, is a radioactive, olive green^[5] cubic^[6] crystalline solid with the formula NpO₂. It is one of two stable oxides of neptunium, the other being neptunium(V) oxide.^[7] It emits both α- and γ-particles.^[4]

Neptunium(IV) oxide

Np4+: __ O2−: __
Names
IUPAC name Neptunium(IV) oxide
Other names Neptunium oxide, neptunium dioxide
Identifiers
CAS Number	12035-79-9 Y
3D model (JSmol)	Interactive image
ChemSpider	34997056
ECHA InfoCard	100.031.651
EC Number	234-830-1
PubChem CID	44148103
InChI InChI=1S/Np.2O/q+4;2*-2 Key: QKUTVYUEUPNRBO-UHFFFAOYSA-N
SMILES [O-2].[O-2].[Np+4]
Properties
Chemical formula	NpO2
Molar mass	269 g/mol
Appearance	Green cubic crystals
Density	11.1 g/cm3
Melting point	2,800 °C; 5,070 °F; 3,070 K[1]
Structure[2]
Crystal structure	Flourite (cubic), cF12
Space group	Fm3m, #225
Lattice constant	a = 543.4 pm
Formula units (Z)	4
Thermochemistry
Std molar entropy (S⦵298)	19.19 ± 0.1 cal·mol−1·K−1 (80.3 ± 0.4 J·mol−1·K−1)[3]
Std enthalpy of formation (ΔfH⦵298)	−256.7 ± 0.6 kcal·mol−1 (−1074 ± 3 kJ·mol−1)[4]
Related compounds
Other anions	Neptunium(III) chloride Neptunium(IV) chloride
Other cations	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). N verify (what is YN ?) Infobox references

Production

Industrially, neptunium dioxide is formed by precipitation of neptunium(IV) oxalate, followed by calcination to neptunium dioxide.^[8]

Production starts with a nitric acid feed solution containing neptunium ions in various oxidation states. First, a hydrazine inhibitor is added to slow any oxidation from standing in air. Then ascorbic acid reduces the feed solution to predominantly neptunium(IV):

2Np⁵⁺ + C₆H₈O₆ → 2Np⁴⁺ + C₆H₆O₆ + 2H⁺

Np⁶⁺ + C₆H₈O₆ → Np⁴⁺ + C₆H₆O₆ + 2H⁺

Addition of oxalic acid precipitates hydrated neptunium oxalate...

Np⁴⁺ + 2H₂C₂O₄ + 6H₂O → Np(C₂O₄)₂.6H₂O(v) + 4H⁺

...which pyrolyzes when heated:^[8]

Np(C₂O₄)₂.6H₂O ^Δ
_→ Np(C₂O₄)₂ ^Δ
_→ NpO₂ + 2CO(g)

Neptunium dioxide can also be formed from precipitation of neptunium(IV) peroxide, but the process is much more sensitive.^[8]

Purification

As a byproduct of nuclear fission reactors, neptunium dioxide can be purified by fluorination, followed by reduction with excess calcium in the presence of iodine.^[4] However, the aforementioned synthesis yields a quite pure solid, with less than 0.3% mass fraction of impurities. Generally, further purification is unnecessary.^[8]

Structure

Due to neptunium's large size, neptunium dioxide has a fluorite structure, with lattice constant a=5.43 Å. Like all fluorite structure materials, it has a space group of Fm3m. Neptunium is eight-coordinate, with a cubic coordination geometry, and oxygen is four-coordinate, with a tetrahedral coordinate geometry.^[9][10]

Other properties

Neptunium dioxide contributes to the α-decay of ²⁴¹Am, reducing its usual half-life by an untested but appreciable amount.^[11] The compound has a low specific heat capacity (900 K, compared with uranium dioxide's specific heat capacity of 1400 K), an abnormality theorized to stem from its 5f electron count.^[12] 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.^[13] 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.^[14]

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.^[14]

Furthermore, a patent for a rocket powered by neptunium dioxide is held by Shirakawa Toshihisa,^[15] but there is little information available into research and production associated with such a product.