Neptunium(IV) oxide
{{chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 429391978
| Name = Neptunium(IV) oxide
| ImageFile = CaF2 polyhedra.png
| ImageSize = 250px
| ImageName = Np4+: __ O2−: __
| ImageFile1 = Neptunium dioxide.png
| ImageSize1 = 100px
| IUPACName = Neptunium(IV) oxide
| OtherNames = Neptunium oxide, neptunium dioxide
| Section1 = {{Chembox Identifiers
| CASNo_Ref = {{cascite|correct|??}}
| CASNo = 12035-79-9
| ChemSpiderID = 34997056
| EINECS = 234-830-1
| PubChem = 44148103
| RTECS =
| StdInChI=1S/Np.2O/q+4;2*-2
| StdInChIKey = QKUTVYUEUPNRBO-UHFFFAOYSA-N
| SMILES = [O-2].[O-2].[Np+4]
}}
| Section2 = {{Chembox Properties
| Formula = NpO2
| MolarMass = 269 g/mol
| Appearance = Green cubic crystals
| Density = 11.1 g/cm3
| Solubility =
| MeltingPtK = 3070
{{cite journal
| last = Böhler
| first = R.
|author2=M. J. Welland |author3=F. De Bruycker |author4=K. Boboridis |author5=A. Janssen |author6=R. Eloirdi |author7=R. J. M. Konings |author8=D. Manara
| year = 2012
| title = Revisiting the melting temperature of NpO2 and the challenges associated with high temperature actinide compound measurements
| journal = Journal of Applied Physics
| volume = 111
| issue = 11
| pages = 113501–113501–8
| publisher = American Institute of Physics
| doi = 10.1063/1.4721655
| bibcode =2012JAP...111k3501B
| url = http://repository.tudelft.nl/islandora/object/uuid%3A0d8eb6d1-9919-448b-b222-d8dec5528ff1/datastream/OBJ/view
}}
| BoilingPt =
}}
| Section3 = {{Chembox Structure
| CrystalStruct = Flourite (cubic), cF12
| SpaceGroup = Fm3m, #225
| LattConst_a = 543.4 pm
| LattConst_b =
| LattConst_c =
| LattConst_alpha =
| LattConst_beta =
| LattConst_gamma =
| MolShape =
| OrbitalHybridisation =
| Dipole =
|UnitCellFormulas = 4
}}
| Section4 = {{Chembox Thermochemistry
| DeltaHf = −256.7 ± 0.6 kcal·mol−1
(−1074 ± 3 kJ·mol−1)
{{cite journal
| last = Huber Jr
| first = Elmer J.
|author2=Charles E. Holley Jr
| date = October 1968| title = Enthalpy of formation of neptunium dioxide
| journal = Journal of Chemical & Engineering Data
| volume = 13
| issue = 4
| pages = 545–546
| doi = 10.1021/je60039a029
}}
| DeltaHc =
| Entropy = 19.19 ± 0.1 cal·mol−1·K−1
(80.3 ± 0.4 J·mol−1·K−1)
{{cite journal
| last = Westrum, Jr.
| first = Edgar F.
| author2 =J. B. Hatcher |author3=Darrell W. Osborne
| date = March 1953| title = The Entropy and Low Temperature Heat Capacity of Neptunium Dioxide
| journal = Journal of Chemical Physics
| volume = 21
| issue = 3
| page = 419
| doi = 10.1063/1.1698923
| bibcode =1953JChPh..21..419W
| url = https://digital.library.unt.edu/ark:/67531/metadc172565/
}}
| HeatCapacity =
}}
| Section7 = {{Chembox Hazards
| FlashPt =
}}
| Section8 = {{Chembox Related
| OtherAnions = Neptunium(III) chloride
Neptunium(IV) chloride
| OtherCations = Protactinium(IV) oxide
Uranium(IV) oxide
Plutonium(IV) oxide
Americium(IV) oxide
}}
}}
Neptunium(IV) oxide, or neptunium dioxide, is a radioactive, olive green{{cite book|last=Patnaik|first=Pradyot|title=Handbook of Inorganic Chemical Compounds|publisher=McGraw-Hill Professional|isbn=0-07-049439-8|page=271|year=2003}} cubic{{cite book|last=Lide|first=D. R.|title=Handbook of Chemistry and Physics 87 ed.|year=1998|publisher=CRC Press|isbn=0-8493-0594-2|page=471}} crystalline solid with the formula NpO2. It is one of two stable oxides of neptunium, the other being neptunium(V) oxide.{{cite journal |date=2018 |title=Experimental thermochemistry of neptunium oxides: Np2O5 and NpO2 |last1=Zhang |first1=Lei |last2=Dzik |first2=Ewa A. |last3=Sigmon |first3=Ginger E. |last4=Szymanowski |first4=Jennifer E.S. |last5=Navrotsky |first5=Alexandra |last6=Burns |first6=Peter C. |journal=Journal of Nuclear Materials |doi=10.1016/j.jnucmat.2017.10.034 |volume=501 |pages=398–403}} It emits both α- and γ-particles.
Production
Industrially, neptunium dioxide is formed by precipitation of neptunium(IV) oxalate, followed by calcination to neptunium dioxide.
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:{{cite journal |last=Porter|first=J. A. |title=Production of Neptunium Dioxide |journal=Industrial & Engineering Chemistry Process Design and Development |year=1964 |volume=4 |issue=3 |pages=289–292|doi=10.1021/i260012a001}} Equations extrapolated from verbal description.
:Np(C2O4)2.6H2O {{su|p=Δ|b=→}} Np(C2O4)2 {{su|p=Δ|b=→}} 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, 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, with lattice constant a=5.43 Å. Like all fluorite structure materials, it has a space group of Fm{{overline|3}}m. Neptunium is eight-coordinate, with a cubic coordination geometry, and oxygen is four-coordinate, with a tetrahedral coordinate geometry.{{cite book |title=Crystal Structures |last=Wyckoff |first=Ralph W.G. |date=1948 |url=https://archive.org/details/dli.ernet.425609 |access-date=22 Apr 2025 |pages=Chapter IV, text page 2}}{{cite book |title=Crystal Structures |last=Wyckoff |first=Ralph W.G. |date=1948 |url=https://archive.org/details/dli.ernet.425609 |access-date=22 Apr 2025 |pages=Chapter IV, table page 14}}
Other properties
Neptunium dioxide contributes to the α-decay of 241Am, reducing its usual half-life by an untested but appreciable amount.{{cite journal |last=Colle |first=J.-Y. |title=(Solid + gas) equilibrium studies for neptunium dioxide |journal=Journal of Chemical Thermodynamics |year=2011 |volume=43 |issue=3 |pages=492–498 |doi=10.1016/j.jct.2010.10.027}} 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.{{cite journal |last1=Serizawa |first1=H. |last2=Arai |first2=Y. |last3=Nakajima |first3=K. |title=The estimation of the heat capacity of NpO2 |journal= The Journal of Chemical Thermodynamics|year=2001 |volume=33 |issue=6 |pages=615–628 |doi=10.1006/jcht.2000.0775}} 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.{{cite journal |last=Hotta |first=T. |title=Microscopic analysis of multipole susceptibility of actinide dioxides: A scenario of multipole ordering in AmO2 |journal=Physical Review B |year=2009 |volume=80 |pages=024408–1–024408–7 |doi=10.1103/PhysRevB.80.024408 |issue=2|arxiv=0906.3607 |bibcode=2009PhRvB..80b4408H |s2cid=119295656 }} 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.{{cite journal |last=Colle |first=J.-Y. |year=2011 |title=(Solid + gas) equilibrium studies for neptunium dioxide |journal=Journal of Chemical Thermodynamics |volume=43 |issue=3 |pages=492–498 |doi=10.1016/j.jct.2010.10.027}}
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,{{cite web|last=Toshihisa|first=Shirakawa|title=Bibliographic data: JP2007040768 (A) - 2007-02-15|url=http://worldwide.espacenet.com/publicationDetails/biblio?DB=EPODOC&II=0&ND=3&adjacent=true&locale=en_EP&FT=D&date=20070215&CC=JP&NR=2007040768A&KC=A|publisher=Espacenet, patent search|access-date=11 April 2012}} but there is little information available into research and production associated with such a product.
References
{{reflist}}
{{Neptunium compounds}}
{{Oxides}}
{{DEFAULTSORT:Neptunium(Iv) Oxide}}