# Neptunium(VI) fluoride

Neptunium(VI) fluoride (NpF$_6$) is the highest fluoride of neptunium, it is also one of seventeen known binary hexafluorides. It is a volatile orange crystalline solid. It is relatively hard to handle, being very corrosive, volatile and radioactive. Neptunium hexafluoride is stable in dry air but reacts vigorously with water.
At normal pressure, it melts at 54.4 °C and boils at 55.18 °C. It is the only neptunium compound that boils at a low temperature. Due to these properties, it is possible to easily separate neptunium from spent fuel.

## Preparation

Neptunium hexafluoride was first prepared in 1943 by American chemist Alan E. Florin, who heated a sample of neptunium(III) fluoride on a nickel filament in a stream of fluorine and condensed the product in a glass capillary tube. Methods of preparation from both neptunium(III) fluoride and neptunium(IV) fluoride were later patented by Glenn T. Seaborg and Harrison S. Brown.

### Standard method

The usual method of preparation is by fluorination of neptunium(IV) fluoride (NpF$_4$) by elemental fluorine (F$_2$) at 500 °C.
    NpF $_4$ + F $_2$ → NpF $_6$
In comparison, uranium hexafluoride (UF$_6$) is formed relatively rapidly from uranium tetrafluoride (UF$_4$) and F$_2$ at 300 °C, while plutonium hexafluoride (PuF$_6$) only begins forming from plutonium tetrafluoride (PuF$_4$) and F$_2$ at 750 °C. This difference allows uranium, neptunium and plutonium to be effectively separated.

### Other methods
#### Using a different starting material

Neptunium hexafluoride can also be obtained by fluorination of neptunium(III) fluoride or neptunium(IV) oxide.
    2 NpF $_3$ + 3 F $_2$ → 2 NpF $_6$
    NpO $_2$ + 3 F $_2$ → NpF $_6$ + O $_2$

#### Using a different fluorine source

The preparation can also be done with the help of stronger fluorinating reagents like bromine trifluoride (BrF$_3$) or bromine pentafluoride (BrF$_5$). These reactions can be used to separate plutonium, since PuF$_4$ does not undergo a similar reaction.
Neptunium dioxide and neptunium tetrafluoride are practically completely converted to volatile neptunium hexafluoride by dioxygen difluoride (O$_2$F$_2$). This works as a gas-solid reaction at moderate temperatures, as well as in anhydrous liquid hydrogen fluoride at −78 °C.
    NpO $_2$ + 3 O $_2$F $_2$ → NpF $_6$ + 4 O $_2$
    NpF $_4$ + O $_2$F $_2$ → NpF $_6$ + O $_2$
These reaction temperatures are markedly different from the high temperatures of over 200 °C previously required to synthesize neptunium hexafluoride with elemental fluorine or halogen fluorides. Neptunyl fluoride (NpO$_2$F$_2$) has been detected by Raman spectroscopy as a dominant intermediate in the reaction with NpO$_2$. Direct reaction of NpF$_4$ with liquid O$_2$F$_2$ led instead to vigorous decomposition of the O$_2$F$_2$ with no NpF$_6$ generation.

## Properties
### Physical properties

Neptunium hexafluoride forms orange orthorhombic crystals that melt at 54.4 °C and boil at 55.18 °C under standard pressure. The triple point is 55.10 °C and 1010 hPa (758 Torr).
The volatility of NpF$_6$ is similar to those of UF$_6$ and PuF$_6$, all three being actinide hexafluorides. The standard molar entropy is 229.1 ± 0.5 J·K$^{−1}$·mol$^{−1}$. Solid NpF$_6$ is paramagnetic, with a magnetic susceptibility of 165·10$^{−6}$ cm$^3$·mol$^{−1}$.

### Chemical properties

Neptunium hexafluoride is stable in dry air. However, it reacts vigorously with water, including atmospheric moisture, to form the water-soluble neptunyl fluoride (NpO$_2$F$_2$) and hydrofluoric acid (HF).
    NpF $_6$ + 2 H $_2$O → NpO $_2$F $_2$ + 4 HF
It can be stored at room temperature in a quartz or pyrex glass ampoule, provided that there are no traces of moisture or gas inclusions in the glass and any remaining HF has been removed. NpF$_6$ is light-sensitive, decomposing to NpF$_4$ and fluorine.
NpF$_6$ forms complexes with alkali metal fluorides: with caesium fluoride (CsF) it forms CsNpF$_6$ at 25 °C, and with sodium fluoride it reacts reversibly to form Na$_3$NpF$_8$. In either case, the neptunium is reduced to Np(V).
    NpF $_6$ + CsF → CsNpF $_6$ + 1/2 F $_2$
    NpF $_6$ + 3 NaF → Na $_3$NpF $_8$ + 1/2 F $_2$
In the presence of chlorine trifluoride (ClF$_3$) as solvent and at low temperatures, there is some evidence of the formation of an unstable Np(IV) complex.
Neptunium hexafluoride reacts with carbon monoxide (CO) and light to form a white powder, presumably containing neptunium pentafluoride (NpF$_5$) and an unidentified substance.$^{:732}$

## Uses

The irradiation of nuclear fuel inside nuclear reactors generates both fission products and transuranic elements, including neptunium and plutonium. The separation of these three elements is an essential component of nuclear reprocessing. Neptunium hexafluoride plays a role in the separation of neptunium from both uranium and plutonium.
In order to separate the uranium (95% of the mass) from spent nuclear fuel, it is first powdered and reacted with elemental fluorine ("direct fluorination"). The resulting volatile fluorides (mainly UF$_6$, small amounts of NpF$_6$) are easily extracted from the non-volatile fluorides of other actinides, like plutonium(IV) fluoride (PuF$_4$), americium(III) fluoride (AmF$_3$), and curium(III) fluoride (CmF$_3$).
The mixture of UF$_6$ and NpF$_6$ is then selectively reduced by pelleted cobalt(II) fluoride, which converts the neptunium hexafluoride to the tetrafluoride but does not react with the uranium hexafluoride, using temperatures in the range of 93 to 204 °C. Another method is using magnesium fluoride, on which the neptunium fluoride is sorbed at 60-70% but not the uranium fluoride.