Neptunium(IV) oxide
Np4+: __ O2−: __
IUPAC name
Neptunium(IV) oxide
Other names
Neptunium oxide, neptunium dioxide
3D model (JSmol)
ECHA InfoCard 100.031.651 Edit this at Wikidata
EC Number
  • 234-830-1
  • InChI=1S/Np.2O/q+4;2*-2
  • [O-2].[O-2].[Np+4]
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]
Flourite (cubic), cF12
Fm3m, #225
a = 543.4 pm
19.19 ± 0.1 cal·mol−1·K−1
(80.3 ± 0.4 J·mol−1·K−1)[3]
−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).
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Neptunium(IV) oxide, or neptunium dioxide, is a radioactive, olive green[5] cubic[6] crystalline solid with the formula NpO2. It emits both α- and γ-particles.[4]


Industrially, neptunium dioxide is formed by precipitation of neptunium(IV) oxalate, followed by calcination to neptunium dioxide.[7]

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:[7]

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.[7]


As a byproduct of nuclear waste, 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.[7]

Other properties

Neptunium dioxide contributes to the α-decay of 241Am, reducing its usual half-life an untested but appreciable amount.[8] 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.[9] Another unique trait of neptunium dioxide is its "mysterious low-temperature ordered phase". Mentioned above, it references an abnormal level of order for an actinitde dioxide complex at low temperature.[10] Further discussion of such topics could indicate useful physical trends in the actinoides.


The neptunium dioxide complex is used as a means of stabilizing, and decreasing the "long term environmental burden"[11] of neptunium as a nuclear fission byproduct. Actinoide-containing nuclear waste will commonly be treated so that various AnO2 (where An = U, Np, Pu, Am, etc.) complexes form. In neptunium dioxide, neptunium is of reduced radio toxicity compared with elemental neptunium and is thus more desirable for storage and disposal. Neptunium dioxide has also been shown to contribute to increased decay rates of radioactive elements, an application which is currently being explored.[11]

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.[11]

Furthermore, a patent for a rocket powered by neptunium dioxide is held by Shirakawa Toshihisa,[12] but there is little information available into research and production associated with such a product.


  1. ^ Böhler, R.; M. J. Welland; F. De Bruycker; K. Boboridis; A. Janssen; R. Eloirdi; R. J. M. Konings; D. Manara (2012). "Revisiting the melting temperature of NpO2 and the challenges associated with high temperature actinide compound measurements". Journal of Applied Physics. American Institute of Physics. 111 (11): 113501–113501–8. Bibcode:2012JAP...111k3501B. doi:10.1063/1.4721655.
  2. ^ Christine Guéneau; Alain Chartier; Paul Fossati; Laurent Van Brutzel; Philippe Martin (2020). "Thermodynamic and Thermophysical Properties of the Actinide Oxides". Comprehensive Nuclear Materials 2nd Ed. 7: 111–154. doi:10.1016/B978-0-12-803581-8.11786-2. ISBN 9780081028667. S2CID 261051636.
  3. ^ Westrum, Jr., Edgar F.; J. B. Hatcher; Darrell W. Osborne (March 1953). "The Entropy and Low Temperature Heat Capacity of Neptunium Dioxide". Journal of Chemical Physics. 21 (3): 419. Bibcode:1953JChPh..21..419W. doi:10.1063/1.1698923.
  4. ^ a b c Huber Jr, Elmer J.; Charles E. Holley Jr (October 1968). "Enthalpy of formation of neptunium dioxide". Journal of Chemical & Engineering Data. 13 (4): 545–546. doi:10.1021/je60039a029.
  5. ^ Patnaik, Pradyot (2003). Handbook of Inorganic Chemical Compounds. McGraw-Hill Professional. p. 271. ISBN 0-07-049439-8.
  6. ^ Lide, D. R. (1998). Handbook of Chemistry and Physics 87 ed. CRC Press. p. 471. ISBN 0-8493-0594-2.
  7. ^ a b c d Porter, J. A. (1964). "Production of Neptunium Dioxide". Industrial & Engineering Chemistry Process Design and Development. 4 (3): 289–292. doi:10.1021/i260012a001. Equations extrapolated from verbal description.
  8. ^ Colle, J.-Y. (2011). "(Solid + gas) equilibrium studies for neptunium dioxide". Journal of Chemical Thermodynamics. 43 (3): 492–498. doi:10.1016/j.jct.2010.10.027.
  9. ^ Serizawa, H.; Arai, Y.; Nakajima, K. (2001). "The estimation of the heat capacity of NpO2". The Journal of Chemical Thermodynamics. 33 (6): 615–628. doi:10.1006/jcht.2000.0775.
  10. ^ Hotta, T. (2009). "Microscopic analysis of multipole susceptibility of actinide dioxides: A scenario of multipole ordering in AmO2". Physical Review B. 80 (2): 024408–1–024408–7. arXiv:0906.3607. Bibcode:2009PhRvB..80b4408H. doi:10.1103/PhysRevB.80.024408. S2CID 119295656.
  11. ^ a b c Colle, J.-Y. (2011). "(Solid + gas) equilibrium studies for neptunium dioxide". Journal of Chemical Thermodynamics. 43 (3): 492–498. doi:10.1016/j.jct.2012.10.027.
  12. ^ Toshihisa, Shirakawa. "Bibliographic data: JP2007040768 (A) - 2007-02-15". Espacenet, patent search. Retrieved 11 April 2012.