Uranium nitrides

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Uranium nitride[1]
Names
IUPAC name
Uranium nitride
Identifiers
ChemSpider
  • InChI=1S/N.U
    Key: MVXWAZXVYXTENN-UHFFFAOYSA-N
Properties
U2N3
Molar mass 518.078 g/mol
Appearance crystalline solid
Density 11300 kg·m−3, solid
Melting point 900 to 1,100 °C (1,650 to 2,010 °F; 1,170 to 1,370 K) (decomposes to UN)
Boiling point Decomposes
0.08 g/100 ml (20 °C)
Structure
Hexagonal, hP5
P-3m1, No. 164
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Uranium nitrides is any of a family of several ceramic materials: uranium mononitride (UN), uranium sesquinitride (U2N3) and uranium dinitride (UN2). The word nitride refers to the −3 oxidation state of the nitrogen bound to the uranium.

Uranium nitride has been considered as a potential

15N is a significant factor to overcome. This is necessary due to the (relatively) high neutron capture cross-section of the far-more-common 14N, which affects the neutron economy of a reactor.[2]

Synthesis

Carbothermic reduction

The common technique for generating UN is

carbothermic reduction of uranium oxide (UO2) in a 2 step method illustrated below.[3][4]

3UO2 + 6C → 2UC + UO2 + 4CO (in argon, > 1450 °C for 10 to 20 hours)
4UC + 2UO2 +3N2 → 6UN + 4CO

Sol-gel

Sol-gel methods and arc melting of pure uranium under nitrogen atmosphere can also be used.[5]

Ammonolysis

Another common technique for generating UN2 is the ammonolysis of uranium tetrafluoride. Uranium tetrafluoride is exposed to ammonia gas under high pressure and temperature, which replaces the fluorine with nitrogen and generates hydrogen fluoride.[6] Hydrogen fluoride is a colourless gas at this temperature and mixes with the ammonia gas.

Hydriding-nitriding

An additional method of UN synthesis employs fabrication directly from metallic uranium. By exposing metallic uranium to hydrogen gas at temperatures in excess of 280 °C, UH3 can be formed.[7] Furthermore, since UH3 has a higher specific volume than the metallic phase, hybridation can be used to physically decompose otherwise solid uranium. Following hybridation, UH3 can be exposed to a nitrogen atmosphere at temperatures around 500 °C, thereby forming U2N3. By additional heating to temperatures above 1150 °C, the sesquinitride can then be decomposed to UN.

2U + 3H2 → 2UH3
2UH3 + 1.5N2 → U2N3
U2N3 → UN + 0.5N2

Use of the

15N (which constitutes around 0.37% of natural nitrogen) is preferable because the predominant isotope, 14N, has a significant neutron absorption cross section which affects neutron economy and, in particular, it undergoes an (n,p) reaction which produces significant amounts of radioactive 14C which would need to be carefully contained and sequestered during reprocessing or permanent storage.[8]

Decomposition

Each uranium dinitride complex is considered to have three distinct compounds present simultaneously because of decomposing of uranium dinitride (UN2) into uranium sesquinitride (U2N3), and then uranium mononitride (UN). Uranium dinitrides decompose to uranium mononitride by the following sequence of reactions:[9]

4UN2 → 2U2N3+ N2
2U2N3 → 4UN +N2

Decomposition of UN2 is the most common method for isolating uranium sesquinitride (U2N3).

Uses

Uranium mononitride is being considered as a potential fuel for

macroscopic restructuring of the fuel, which limits fuel lifetime.[4]

Molecular and crystal structure

The uranium dinitride (UN2) compound has a

face-centered cubic crystal structure of the calcium fluoride (CaF2) type with a space group of Fm3m.[13]
Nitrogen forms triple bonds on each side of uranium forming a linear structure.[14][15]

α-(U2N3) has a

body-centered cubic crystal structure of the (Mn2O3) type with a space group of Ia3 .[13]

UN has a

NaCl type.[14][16]
The
p orbitals on the nitrogen.[14][17] N forms a triple bond with uranium creating a linear structure.[15]

  • Uranium dinitride
    Uranium dinitride
  • Uranium sesquinitride
    Uranium sesquinitride
  • Uranium mononitride
    Uranium mononitride

Uranium nitrido derivatives

Recently, there have been many developments in the synthesis of complexes with terminal uranium nitride (–U≡N) bonds. In addition to radioactive concerns common to all uranium chemistry, production of uranium nitrido complexes has been slowed by harsh reaction conditions and solubility challenges. Nonetheless, syntheses of such complexes have been reported in the past few years, for example the three shown below among others.[18][19] Other U≡N compounds have also been synthesized or observed with various structural features, such as bridging nitride ligands in di-/polynuclear species, and various oxidation states.[20][21]

[N(n-Bu)4] [(C6F5)3B−N≡U(Nt-BuAr)3][22]
N≡UF3[23]
[Na(12-crown-4)2] [N≡U−N(CH2CH2Ntips)3][24]

See also

References

  1. doi:10.1016/0022-3115(88)90029-3
    .
  2. doi:10.1016/j.pnucene.2012.11.001
    .
  3. ISSN 0022-3115
    .
  4. ^
    ISSN 0094-243X
    .
  5. ^ Ganguly, C.; Hegde, P. J. Sol-Gel Sci. Technol.. 1997, 9, 285.
  6. ^ Silva, G. W. C.; Yeamans, C. B.; Ma, L.; Cerefice, G. S.; Czerwinski, K. R.; Sarrelberger, A. P. Chem. Mater.. 2008, 20, 3076.
  7. ^ urn:nbn:se:kth:diva-35249: Manufacturing methods for (U-Zr)N-fuels
  8. ^ a b Matthews, R. B.; Chidester, K. M.; Hoth, C. W.; Mason, R. E.; Petty, R. L. Journal of Nuclear Materials. '1988, 151(3), 345.
  9. ^
    PMID 19845318
    .
  10. ^ Staff (2009-11-20). "Hyperion launches U2N3-fuelled, Pb-Bi-cooled fast reactor". Nuclear Engineering International. Global Trade Media, a division of Progressive Media Group Ltd.
  11. ^ "Simple method for producing a stable form of uranium nitride". Advanced Ceramics Report. International Newsletters. August 1, 2012. [R]esearcher ... Stephen Liddle, says: '... it could help... extract and separate the 2-3% of the highly radioactive material in nuclear waste.'
  12. ^ Mizutani, A.; Sekimoto, H. Ann. Nucl. Energy. 2005, 25(9), 623–638.
  13. ^ a b Rundle, R. E.; Baenziger, N. C.; Wilson, A. S.; McDonald, R. A. J. Am. Chem Soc.. 1948, 70, 99.
  14. ^ a b c Weck P. F., Kim E., Balakrishnan N., Poineau F., Yeamans C. B., and Czerwinski K. R. Chem. Phys. Lett.. 2007, 443, 82.
    doi:10.1016/j.cplett.2007.06.047
  15. ^ a b Wang, X.; Andrews, L.; Vlaisavljevich, B.; Gagliardi, L. Inorganic Chemistry. 2011, 50(8), 3826–3831. doi:10.1021/ic2003244
  16. ^ Mueller, M. H.; Knott, H. W.Acta Crystallogr.. 1958, 11, 751–752. doi:10.1107/S0365110X58002061
  17. ^ Évarestov, R. A., Panin, A. I., & Losev, M. V. Journal of Structural Chemistry. 2008, 48, 125–135.
  18. ^ Nocton, G.; Pécaut, J.; Mazzanti, M. A Nitrido-Centered Uranium Azido Cluster Obtained from a Uranium Azide. Angew. Chem. Int. Ed. 2008, 47 (16), 3040–3042.
    doi:10.1002/anie.200705742
  19. doi:10.1038/nchem.705
  20. doi:10.1021/ja910364u
  21. doi:10.1126/science.1116452
  22. doi:10.1021/ja8095812
  23. doi:10.1002/anie.200801120
  24. doi:10.1126/science.1223488

External links
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