Neptunium (VI) fluoride

Structural formula
Crystal system	orthorhombic
Space group	Pnma (No. 62)
Lattice parameters	a = 990.9 pm b = 899.7 pm c = 520.2 pm
General
Surname	Neptunium (VI) fluoride
other names	Neptunium hexafluoride
Molecular formula	NpF 6
Brief description	orange crystalline solid
External identifiers / databases
CAS number 14521-05-2 PubChem 19695135 Wikidata Q1977886
properties
Molar mass	351.04 g mol −1 ( 237 Np)
Physical state	firmly
Melting point	54.4 ° C
boiling point	55.18 ° C
Hazard and safety information
Radioactive
GHS hazard labeling no classification available
Thermodynamic properties
ΔH f 0	−1970.0 ± 20.0 kJ mol −1
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Neptunium (VI) fluoride (NpF ₆ ), usually called neptunium hexafluoride , is a chemical compound made up of the elements neptunium and fluorine . It is an orange crystalline solid that is highly volatile , radioactive and corrosive . Neptunium hexafluoride is stable in dry air, but reacts violently with water. In most cases it is obtained from neptunium (IV) fluoride  (NpF ₄ ) by reaction with elemental fluorine (F ₂ ).

At normal pressure it melts at 54.4 ° C and boils at 55.18 ° C. It is the only neptunium compound that can easily be converted into the gas phase. Due to these properties, separation of neptunium from spent fuel elements is possible. As a result, interest in its representation and in the detailed investigation of its properties grew rapidly.

presentation

Neptunium hexafluoride was first presented in 1943 by the American chemist Alan E. Florin . He heated a sample of neptunium (III) fluoride in a nickel crucible in a flow of fluorine gas and condensed the volatile neptunium hexafluoride in a thin-walled glass capillary. The preparation of neptunium hexafluoride from both neptunium (III) fluoride and neptunium (IV) fluoride was published and submitted by Glenn T. Seaborg and Harrison S. Brown on October 17, 1947 in the form of a patent specification.

Standard method

The common method for the preparation of neptunium hexafluoride is the conversion of neptunium tetrafluoride  (NpF ₄ ) with elemental fluorine  (F ₂ ) at 500 ° C.

${\displaystyle {\ce {NpF4 + F2 -> NpF6}}}$

Uranium hexafluoride  (UF ₆ ) is formed relatively rapidly even at 300 ° C from uranium tetrafluoride  (UF ₄ ) and fluorine (F ₂ ), while plutonium hexafluoride  (PuF ₆ ) only at 750 ° C of plutonium tetrafluoride  (PuF ₄ ), and F ₂ is formed. In this way, uranium, neptunium and plutonium can be effectively separated from each other.

With other raw materials

Neptunium hexafluoride has also been produced by fluorinating neptunium (III) fluoride or neptunium (IV) oxide .

${\displaystyle {\ce {2 NpF3 + 3 F2 -> 2 NpF6}}}$

${\displaystyle {\ce {NpO2 + 3 F2 -> NpF6 + O2}}}$

With other fluorine compounds

It can also be represented using strong fluorinating reagents such as bromine trifluoride  (BrF ₃ ) or bromine pentafluoride  (BrF ₅ ). These reactions can be used specifically for the separation of plutonium, since plutonium tetrafluoride  (PuF ₄ ) does not enter into any corresponding reactions.

Neptunium dioxide and tetrafluoride are almost completely converted to volatile neptunium hexafluoride by dioxygen difluoride  (O ₂ F ₂ ). This is possible both in gas-solid reactions at moderate temperatures and in liquid anhydrous hydrogen fluoride at −78 ° C:

${\displaystyle {\ce {NpO2 + 3 O2F2 -> NpF6 + 4 O2}}}$

${\displaystyle {\ce {NpF4 + O2F2 -> NpF6 + O2}}}$

These reaction temperatures are in clear contrast to the otherwise higher temperatures of over 200 ° C., which are previously required for the preparation of NpF ₆ when using elemental fluorine or halogen fluorides . In this reaction, neptunyl (VI) fluoride  (NpO ₂ F ₂ ) was identified by Raman spectroscopy as an essential intermediate in the reaction with neptunium dioxide. A direct reaction of NpF ₄ with liquid O ₂ F ₂ , on the other hand, leads to vigorous decomposition of the O ₂ F ₂ with little to no conversion to NpF ₆ .

properties

Physical Properties

Parameters for the Antoine equation according to lg p = A − B · T + C · lg (T); for 0-55.1 ° C
A.	B.	C.
18.48130	2892.0	−2.6990
after lg p = A − B · T + C · lg (T); for 55.1-76.82 ° C
A.	B.	C.
0.01023	1191.1	2.5825

Neptunium hexafluoride forms orange orthorhombic crystals. They melt at normal pressure  (1013.25  hPa ) at 54.4 ° C. The boiling point is 55.18 ° C.

The triple point at which the three phases solid, liquid and gaseous are in equilibrium is at a temperature of 55.10 ° C at a pressure of 1010 hPa (758 Torr). This means that below this pressure - which is only slightly below normal pressure - solid neptunium hexafluoride changes directly into the gaseous state when heated by sublimation .

The volatility of NpF ₆ is similar to that of uranium hexafluoride  (UF ₆ ) and plutonium hexafluoride  (PuF ₆ ); together they belong to the three known hexafluorides of the actinide elements . The Bildungsentropie  (S ⁰ _m is) for NpF ₆ 229.1 ± 0.5  J · K ^-1 · mol ^-1 . Fixed NpF ₆ is paramagnetic ; the molar magnetic susceptibility χ _mol is 165 · 10 ⁻⁶  cm ³ · mol ⁻¹ .

Crystal and molecular structure

Neptunium hexafluoride is a covalent compound and not a salt. It crystallizes in the orthorhombic crystal system in the space group  Pnma (No. 62) with the lattice parameters a  = 990.9  pm , b  = 899.7 pm and c  = 520.2 pm with four formula units per unit cell . In the gaseous state it consists of regular octahedral molecules ( O _h ) with a uniform Np – F bond length of 198.1 pm.

• 

  Unit cell of neptunium hexafluoride

• 

  Bond length and angle in gaseous neptunium hexafluoride

Spectroscopic properties

Neptunium hexafluoride has six fundamental vibrations . ν ₁ , ν ₂ and ν ₃ are stretching vibrations and ν ₄ , ν ₅ and ν ₆ are bending vibrations . Of these, ν ₁ , ν ₂ and ν _{5 are} Raman-active , ν ₃ and ν _{4 are} IR-active , ν ₆ is IR- and Raman-inactive.

Fundamental vibration	ν 1	ν 2	ν 3	ν 4	ν 5	ν 6
Term symbol	A 1g	E g	F 1u	F 1u	F 2g	F 2u
Wave number (cm −1 )	654	535	624	198.6	208	(164)
IR active	-	-	+	+	-	-
Raman active	+	+	-	-	+	-

Chemical properties

Neptunium hexafluoride is stable in dry air. On the other hand, it reacts very violently with water (due to humidity in the air), producing the water-soluble neptunyl (VI) fluoride  (NpO ₂ F ₂ ) and hydrogen fluoride  (HF).

${\displaystyle {\ce {NpF6 + 2 H2O -> NpO2F2 + 4 HF}}}$

It can be stored indefinitely at room temperature in quartz or PYREX ampoules if it is ensured that there are no traces of moisture, the glass itself is free of all gas inclusions and any hydrogen fluoride  (HF) that may be present has been completely removed.

NpF ₆ and PuF ₆ are light sensitive and decompose to the tetrafluorides and fluorine.

NpF ₆ forms with alkali fluorides fluoro . With cesium fluoride  (CsF) it forms CsNpF ₆ at 25 ° C , with sodium fluoride  (NaF) it reacts reversibly to form Na ₃ NpF ₈ . In both cases there is a reduction from hexavalent neptunium to pentavalent.

${\displaystyle {\ce {NpF6 + CsF -> CsNpF6 + 1/2 F2}}}$

${\displaystyle {\ce {NpF6 + 3 NaF -> Na3NpF8 + 1/2 F2}}}$

In the presence of chlorine trifluoride  (ClF ₃ ) as solvent and at lower temperatures, there are signs of the formation of an unstable Np (VI) complex.

The hydrolysis of neptunium hexafluoride in an almost anhydrous hydrogen fluoride solution leads to the oxide fluoride NpOF ₄ . It has the same structure as the trigonal shape of the UOF ₄ . Oxidation of the NpOF ₄ by krypton difluoride  (KrF ₂ ) in anhydrous HF to the maximum possible oxidation state , however, does not take place Np (VII).

Neptunium hexafluoride reacts with carbon monoxide  (CO) and light to form a fine white powder, which consists of neptunium pentafluoride  (NpF ₅ ) and other, unidentified material.

use

In the irradiation of fuel elements in nuclear power plants in addition to arise fission products and transuranic elements , including neptunium and plutonium. The separation of uranium, neptunium and plutonium is necessary for reprocessing . Due to its volatility, neptunium hexafluoride primarily plays a role in the separation of neptunium from both uranium and plutonium.

In order to separate the uranium (95% of the total mass) from spent fuel elements, it was proposed to finely shred the material and treat it with elemental fluorine ("direct fluorination"). The resulting volatile fluorides (mainly UF ₆ and small amounts of NpF ₆ ) can easily be removed from the non-volatile fluorides, e.g. B.  Separate plutonium (IV) fluoride (PuF ₄ ), americium (III) fluoride  (AmF ₃ ), curium (III) fluoride  (CmF ₃ ), as well as the fluorides of most of the cleavage products .

From mixtures of uranium and neptunium hexafluoride, the neptunium hexafluoride is selectively reduced to neptunium tetrafluoride with pelleted cobalt (II) fluoride at temperatures in the range from 93 to 204 ° C. The uranium hexafluoride present in the mixture does not enter into this reaction. When using magnesium fluoride at 100 ° C, neptunium hexafluoride is absorbed by 60-70%, while uranium hexafluoride does not react.

safety instructions

On contact with moisture , neptunium hexafluoride forms hydrofluoric acid , which causes chemical burns on the skin and the mucous membranes of the respiratory tract and can lead to pneumonitis and pulmonary edema and thus death in the event of prolonged exposure . Like all neptunium compounds, it is also radioactive . The activity depends on the isotopic composition of the neptunium. The mostly questionable ²³⁷ Np has a half-life of 2.144 million years and is an α-emitter . ²³⁵ Np and ²³⁶ Np have half-lives of 396.1 days and 154,000 years, respectively, and are therefore significantly more active; both decay primarily through electron capture .

Classifications according to the GHS regulation are not available because they only include chemical hazard, which plays a completely subordinate role compared to the hazards based on radioactivity .

literature

• Zenko Yoshida, Stephen G. Johnson, Takaumi Kimura, John R. Krsul: Neptunium , in: Lester R. Morss, Norman M. Edelstein, Jean Fuger (eds.): The Chemistry of the Actinide and Transactinide Elements , Springer, Dordrecht 2006 ; ISBN 1-4020-3555-1 , pp. 699-812 ( doi : 10.1007 / 1-4020-3598-5_6 ; full text (PDF) ).
• C. Keller: The chemistry of neptunium , in: Fortschr. chem. Forsch. , 1969/70 , 13/1 , pp. 1–124 ( doi: 10.1007 / BFb0051170 ).
• GL Goodman, Mark Fred: Electronic Structure of Neptunium Hexafluoride , in: J. Chem. Phys. , 1959 , 30 , pp. 849-850 ( doi: 10.1063 / 1.1730060 ; abstract ).
• B. Frlec: The density of liquid neptunium hexafluoride , in: Journal of Inorganic and Nuclear Chemistry , 1967 , 29  (7), pp. 1804-1805 ( doi: 10.1016 / 0022-1902 (67) 80227-6 ; abstract ).

Individual evidence

1. ↑ ^{a b c d} C. Keller: The chemistry of Neptunium , in: Fortschr. chem. Forsch. , 1969/70 , 13/1 , pp. 1–124, here: pp. 71–75.
2. ↑ The hazards emanating from radioactivity do not belong to the properties to be classified according to the GHS labeling. With regard to other hazards, this substance has either not yet been classified or a reliable and citable source has not yet been found.
3. ↑ ^{a b} Zenko Yoshida, Stephen G. Johnson, Takaumi Kimura, John R. Krsul: Neptunium , p. 736 ( limited preview in the Google book search).
4. ^ AE Florin, Report MUC-GTS-2165 (1943).
5. ^ ^{A b} Sherman Fried, Norman Davidson: The Preparation of Solid Neptunium Compounds , in: J. Am. Chem. Soc. , 1948 , 70  (11), pp. 3539-3547 ( doi: 10.1021 / ja01191a003 ).
6. ↑ US Patent 2982604: Preparation of Neptunium Hexafluoride , April 25, 1961 ( PDF ).
7. ^ ^{A b c d} John G. Malm, Bernard Weinstock, E. Eugene Weaver: The Preparation and Properties of NpF ₆ ; a Comparison with PuF ₆ , in: J. Phys. Chem. , 1958 , 62  (12), pp. 1506-1508 ( doi: 10.1021 / j150570a009 ).
8. ↑ LE Trevorrow, TJ Gerding, MJ Steindler: Laboratory Investigations in Support of Fluid-bed Fluoride Volatility Processes, Part XVII, The Fluorination of Neptunium (IV) fluoride and Neptunium (IV) oxide (Argonne National Laboratory Report ANL-7385); January 1, 1968 ( doi: 10.2172 / 4492135 ; abstract ; PDF ).
9. ^ LE Trevorrow, TJ Gerding, MJ Steindler: The Fluorination of Neptunium (IV) fluoride and Neptunium (IV) oxide , in: Journal of Inorganic and Nuclear Chemistry , 1968 , 30  (10), pp. 2671–2677 ( doi: 10.1016 / 0022-1902 (68) 80394-X ).
10. ^ ^{A b c} P. Gary Eller, Larned B. Asprey, Scott A. Kinkead, Basil I. Swanson, Richard J. Kissane: Reactions of Dioxygen Difluoride with Neptunium Oxides and Fluorides , in: Journal of Alloys and Compounds , 1998 , 269  (1-2), pp. 63-66 ( doi: 10.1016 / S0925-8388 (98) 00005-X ).
11. ↑ Zenko Yoshida, Stephen G. Johnson, Takaumi Kimura, John R. Krsul: Neptunium , p. 733 ( limited preview in Google Book Search).
12. ^ Clyde A. Hutchison , Bernard Weinstock: Paramagnetic Resonance Absorption in Neptunium Hexafluoride , in: J. Chem. Phys. , 1960 , 32 , pp. 56-61 ( doi: 10.1063 / 1.1700947 ; abstract ).
13. ^ Clyde A. Hutchison, Tung Tsang, Bernard Weinstock: Magnetic Susceptibility of Neptunium Hexafluoride in Uranium Hexafluoride , in: J. Chem. Phys. , 1962 , 37 , pp. 555-562 ( doi: 10.1063 / 1.1701373 ; abstract ).
14. ↑ ^{a b c} Gmelin's Handbook of Inorganic Chemistry , System No. 71, Transurane, Part C, pp. 108–114.
15. ^ ^{A b} Masao Kimura, Werner Schomaker, Darwin W. Smith, Bernard Weinstock: Electron-Diffraction Investigation of the Hexafluorides of Tungsten, Osmium, Iridium, Uranium, Neptunium, and Plutonium , in: J. Chem. Phys. , 1968 , 48  (8), pp. 4001-4012 ( doi: 10.1063 / 1.1669727 ).
16. ↑ Earl L. Grasner, Boris Frlec: Raman Spectrum of Neptunium Hexafluoride , in: J. Chem. Phys. , 1968 , 49  (11), pp. 5135-5137 ( doi: 10.1063 / 1.1670010 ; abstract ).
17. ↑ B. Weinstock, EE Weaver, JG Malm: Vapor Pressures of NpF ₆ and PuF ₆ ; Thermodynamic Calculations with UF ₆ , NpF ₆ and PuF ₆ , in: Journal of Inorganic and Nuclear Chemistry , 1959 , 11  (2), pp. 104-114 ( doi: 10.1016 / 0022-1902 (59) 80054-3 ).
18. ↑ KC Kim, RN Mulford: Vibrational Properties of Actinide (U, Np, Pu, Am) Hexafluoride Molecules , in: Journal of Molecular Structure: THEOCHEM , 1990 , 207  (3-4), pp. 293-299 ( doi: 10.1016 / 0166-1280 (90) 85031-H ).
19. ^ JC Eisenstein, MHL Pryce: Theory of the Magnetic and Spectroscopic Properties of Neptunium Hexafluoride , in: Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences , (April 5, 1960), Vol. 255, No. 1281, pp. 181-198 ( doi: 10.1098 / rspa.1960.0061 ; abstract ; abstract ).
20. ↑ MJ Steindler, TJ Gerding: The absorption spectrum of neptunium hexafluoride , in: Spectrochimica Acta , 1966 , 22  (6), pp. 1197-1200 ( doi: 10.1016 / 0371-1951 (66) 80212-6 ; abstract ).
21. ^ ^{A b} R. D. Peacock, Norman Edelstein: Some Reactions of Neptunium Hexafluoride , in: Journal of Inorganic and Nuclear Chemistry , 1976 , 38  (4), pp. 771-773 ( doi: 10.1016 / 0022-1902 (76) 80353-3 ).
22. ↑ LeVerne E. Trevorrow, TJ Gerding, Martin J. Steindler: The Reaction of Neptunium Hexafluoride with Sodium Fluoride , in: Inorg. Chem. , 1968 , 7  (11), pp. 2226-2229 ( doi: 10.1021 / ic50069a010 ).
23. ↑ Zenko Yoshida, Stephen G. Johnson, Takaumi Kimura, John R. Krsul: Neptunium , p. 732 ( limited preview in Google Book Search).
24. ↑ Jan Uhlíř, Martin Marečeka: Fluoride Volatility Method for Reprocessing of LWR and FR Fuels , in: Journal of Fluorine Chemistry , 2009 , 130  (1), pp. 89–93 ( doi: 10.1016 / j.jfluchem.2008.07.002 ) .
25. ↑ US Patent 3615267: Separation of Neptunium from Uranium Hexafluoride Containing the Same , October 23, 1971 ( PDF ).
26. ↑ Tsuyoshi Nakajima, Henri Groult (ed.): Fluorinated Materials for Energy Conversion , Elsevier, Amsterdam 2005, p. 559 ( limited preview in the Google book search).
27. ↑ G. Audi, O. Bersillon, J. Blachot, AH Wapstra: The NUBASE evaluation of nuclear and decay properties. In: Nuclear Physics. Volume A 729, 2003, pp. 3–128, here: pp. 117–118. doi : 10.1016 / j.nuclphysa.2003.11.001 . ( PDF ; 1.0 MB).

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