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Neptunium is a chemical element; it has chemical symbol Np and atomic number 93. A radioactivity actinide metal, neptunium is the first transuranic element. It is named after Neptune, the planet beyond Uranus in the Solar System, which uranium is named after. A neptunium atom has 93 and 93 electrons, of which seven are . Neptunium metal is silvery and when exposed to air. The element occurs in three allotrope forms and it normally exhibits five , ranging from +3 to +7. Like all actinides, it is radioactive, poisonous, pyrophoricity, and capable of accumulating in , which makes the handling of neptunium dangerous.
Although many false claims of its discovery were made over the years, the element was first synthesized by Edwin McMillan and Philip H. Abelson at the Berkeley Radiation Laboratory in 1940. Since then, most neptunium has been and still is produced by neutron irradiation of uranium in nuclear reactors. The vast majority is generated as a by-product in conventional nuclear power reactors. While neptunium itself has no commercial uses at present, it is used as a precursor for the formation of plutonium-238, which is in turn used in radioisotope thermal generators to provide electricity for spacecraft. Neptunium has also been used in neutron detector of high-energy .
The longest-lived isotope of neptunium, neptunium-237, is a by-product of and plutonium production. This isotope, and the isotope neptunium-239, are also found in trace amounts in uranium ores due to neutron capture and beta decay.
Neptunium is found in at least three . Some claims of a fourth allotrope have been made, but they are so far not proven.Yoshida et al., p. 718. This multiplicity of allotropes is common among the . The crystal structures of neptunium, protactinium, uranium, and plutonium do not have clear analogs among the and are more similar to those of the 3d . (2025). 9785769525339, Academy. ISBN 9785769525339
| + Allotropes of neptunium |
α-neptunium takes on an orthorhombic structure, resembling a highly distorted body-centered cubic structure.Lemire, R. J. et al., Chemical Thermodynamics of Neptunium and Plutonium, Elsevier, Amsterdam, 2001. Each neptunium atom is coordinated to four others and the Np–Np bond lengths are 260 pm.Yoshida et al., p. 719. It is the densest of all the actinides and the fifth-densest of all naturally occurring elements, behind only rhenium, platinum, iridium, and osmium.Theodore Gray. The Elements. Page 215. α-neptunium has properties, such as strong and a high electrical resistivity, and its metallic physical properties are closer to those of the than the true metals. Some allotropes of the other actinides also exhibit similar behaviour, though to a lesser degree.Hindman J. C. 1968, "Neptunium", in C. A. Hampel (ed.), The encyclopedia of the chemical elements, Reinhold, New York, pp. 434. The densities of different isotopes of neptunium in the alpha phase are expected to be observably different: α-235Np should have density 20.303 g/cm3; α-236Np, density 20.389 g/cm3; α-237Np, density 20.476 g/cm3.
β-neptunium takes on a distorted tetragonal close-packed structure. Four atoms of neptunium make up a unit cell, and the Np–Np bond lengths are 276 pm. γ-neptunium has a body-centered cubic structure and has Np–Np bond length of 297 pm. The γ form becomes less stable with increased pressure, though the melting point of neptunium also increases with pressure. The β-Np/γ-Np/liquid triple point occurs at 725 °C and 3200 MPa.
One neptunium-based superconductor alloy has been discovered with formula Nppalladium5Al2. This occurrence in neptunium compounds is somewhat surprising because they often exhibit strong magnetism, which usually destroys superconductivity. The alloy has a tetragonal structure with a superconductivity transition temperature of −268.3 °C (4.9 K).
The isotopes of neptunium range in atomic weight from 219.032 u (219Np) to 244.068 u (244Np), though 221Np has not yet been reported. Most of the isotopes that are lighter than the most stable one, 237Np, decay primarily by electron capture although a sizable number, most notably 229Np and 230Np, also exhibit various levels of decay via alpha emission to become protactinium. 237Np itself, being the beta-stable isobar of mass number 237, decays almost exclusively by alpha emission into 233Pa, with very rare (occurring only about once in trillions of decays) spontaneous fission and cluster decay (emission of 30Mg to form 207Tl). All of the known isotopes except one that are heavier than this decay exclusively via beta emission. The lone exception, 240mNp, exhibits a rare (>0.12%) decay by isomeric transition in addition to beta emission. 237Np eventually decays to form bismuth-209 and thallium-205, unlike most other common heavy nuclei which decay into isotopes of lead. This decay chain is known as the neptunium series. This decay chain had long been extinct on Earth due to the short half-lives of all of its isotopes above bismuth-209, but is now being resurrected thanks to artificial production of neptunium on the tonne scale. (2025). 9783527306732, Wiley. ISBN 9783527306732
The isotopes neptunium-235, -236, and -237 are predicted to be fissile; only neptunium-237's fissionability has been experimentally shown, with the critical mass being about 60 kg, only about 10 kg more than that of the commonly used uranium-235. Calculated values of the critical masses of neptunium-235, -236, and -237 respectively are 66.2 kg, 6.79 kg, and 63.6 kg: the neptunium-236 value is even lower than that of plutonium-239. In particular, 236Np also has a low neutron cross section. Despite this, a neptunium atomic bomb has never been built: uranium and plutonium have lower critical masses than 235Np and 237Np, and 236Np is difficult to purify as it is not found in quantity in spent nuclear fuel and is nearly impossible to separate in any significant quantities from 237Np.
Trace amounts of the neptunium isotopes neptunium-237 and -239 are found naturally as from transmutation reactions in .Emsley, pp. 345–347. 239Np and 237Np are the most common of these isotopes; they are directly formed from neutron capture by uranium-238 atoms. These neutrons come from the spontaneous fission of uranium-238, naturally neutron-induced fission of uranium-235, cosmic ray spallation of nuclei, and light elements absorbing and emitting a neutron. The half-life of 239Np is very short, although the detection of its much longer-lived daughter product 239Pu in nature in 1951 definitively established its natural occurrence. In 1952, 237Np was identified and isolated from concentrates of uranium ore from the Belgian Congo: in these minerals, the ratio of neptunium-237 to uranium is less than or equal to about 10−12 to 1. Additionally, 240Np must also occur as an intermediate decay product of 244Pu, which has been detected in meteorite dust in marine sediments on Earth.
Most neptunium (and plutonium) now encountered in the environment is due to atmospheric nuclear explosions that took place between the detonation of the Trinity test in 1945 and the ratification of the Partial Nuclear Test Ban Treaty in 1963. The total amount of neptunium released by these explosions and the few atmospheric tests that have been carried out since 1963 is estimated to be around 2500 kg. The overwhelming majority of this is composed of the long-lived isotopes 236Np and 237Np since even the moderately long-lived 235Np (half-life 396 days) would have decayed to less than one-billionth (10−9) its original concentration over the intervening decades. An additional very small amount of neptunium, produced by neutron irradiation of natural uranium in nuclear reactor cooling water, is released when the water is discharged into rivers or lakes. The concentration of 237Np in seawater is approximately 6.5 × 10−5 millibecquerels per liter: this concentration is between 0.1% and 1% that of plutonium.
Once released in the surface environment, in contact with atmospheric oxygen, neptunium generally oxidation fairly quickly, usually to the +4 or +5 state. Regardless of its oxidation state, the element exhibits much greater mobility than the other actinides, largely due to its ability to readily form aqueous solutions with various other elements. In one study comparing the diffusion rates of neptunium(V), plutonium(IV), and americium(III) in sandstone and limestone, neptunium penetrated more than ten times as well as the other elements. Np(V) will also react efficiently in pH levels greater than 5.5 if there are no present and in these conditions it has also been observed to readily bond with quartz. It has also been observed to bond well with goethite, ferric oxide colloids, and several clays including kaolinite and smectite. Np(V) does not bond as readily to soil particles in mildly acidic conditions as its fellow actinides americium and curium by nearly an order of magnitude. This behavior enables it to migrate rapidly through the soil while in solution without becoming fixed in place, contributing further to its mobility.Atwood, section 4. Np(V) is also readily absorbed by concrete, which because of the element's radioactivity is a consideration that must be addressed when building nuclear waste storage facilities. When absorbed in concrete, it is Redox to Np(IV) in a relatively short period of time. Np(V) is also reduced by if they are present on the surface of goethite, hematite, and magnetite. Np(IV) is less mobile and efficiently Sorption by tuff, granodiorite, and bentonite; although uptake by the latter is most pronounced in mildly acidic conditions. It also exhibits a strong tendency to bind to colloid, an effect that is enhanced when in surface soil with high clay content. The behavior provides an additional aid in the element's observed high mobility.Atwood, section 1.
Up to and after the discovery of the final component of the atomic nucleus, the neutron in 1932, most scientists did not seriously consider the possibility of elements heavier than uranium. While nuclear theory at the time did not explicitly prohibit their existence, there was little evidence to suggest that they did. However, the discovery of induced radioactivity by Irène and Frédéric Joliot-Curie in late 1933 opened up an entirely new method of researching the elements and inspired a small group of Italian scientists led by Enrico Fermi to begin a series of experiments involving neutron bombardment. Although the Joliot-Curies' experiment involved bombarding a sample of 27Al with to produce the radioactive 30P, Fermi realized that using neutrons, which have no electrical charge, would most likely produce even better results than the positively charged alpha particles. Accordingly, in March 1934 he began systematically subjecting all of the then-known elements to neutron bombardment to determine whether others could also be induced to radioactivity.Rhodes, pp. 201–202.Rhodes, pp. 209–210.
After several months of work, Fermi's group had tentatively determined that lighter elements would disperse the energy of the captured neutron by emitting a proton or alpha particle and heavier elements would generally accomplish the same by emitting a gamma ray. This latter behavior would later result in the beta decay of a neutron into a proton, thus moving the resulting isotope one place up the periodic table. When Fermi's team bombarded uranium, they observed this behavior as well, which strongly suggested that the resulting isotope had an atomic number of 93. Fermi was initially reluctant to publicize such a claim, but after his team observed several unknown half-lives in the uranium bombardment products that did not match those of any known isotope, he published a paper entitled Possible Production of Elements of Atomic Number Higher than 92 in June 1934. For element 93, he proposed the name ausenium (atomic symbol Ao) after the Greek name Ausonia for Italy.Enrico Fermi, Artificial radioactivity produced by neutron bombardment, Nobel Lecture, December 12, 1938.
Several theoretical objections to the claims of Fermi's paper were quickly raised; in particular, the exact process that took place when an atom Neutron capture was not well understood at the time. This and Fermi's accidental discovery three months later that nuclear reactions could be induced by slow neutrons cast further doubt in the minds of many scientists, notably Aristid von Grosse and Ida Noddack, that the experiment was creating element 93. While von Grosse's claim that Fermi was actually producing protactinium (element 91) was quickly tested and disproved, Noddack's proposal that the uranium had been shattered into two or more much smaller fragments was simply ignored by most because existing nuclear theory did not include a way for this to be possible. Fermi and his team maintained that they were in fact synthesizing a new element, but the issue remained unresolved for several years.Hoffman, pp. 120–123.Rhodes, pp. 210–220.
Although the many different and unknown radioactive half-lives in the experiment's results showed that several nuclear reactions were occurring, Fermi's group could not prove that element 93 was being produced unless they could isolate it chemically. They and many other scientists attempted to accomplish this, including Otto Hahn and Lise Meitner who were among the best radiochemists in the world at the time and supporters of Fermi's claim, but they all failed. Much later, it was determined that the main reason for this failure was because the predictions of element 93's chemical properties were based on a periodic table which lacked the actinide series. This arrangement placed protactinium below tantalum, uranium below tungsten, and further suggested that element 93, at that point referred to as eka-rhenium, should be similar to the group 7 elements, including manganese and rhenium. Thorium, protactinium, and uranium, with their dominant oxidation states of +4, +5, and +6 respectively, fooled scientists into thinking they belonged below hafnium, tantalum, and tungsten, rather than below the lanthanide series, which was at the time viewed as a fluke, and whose members all have dominant +3 states; neptunium, on the other hand, has a much rarer, more unstable +7 state, with +4 and +5 being the most stable. Upon finding that plutonium and the other transuranic elements also have dominant +3 and +4 states, along with the discovery of the f-block, the actinide series was firmly established.Rhodes, pp. 221–222.Rhodes, p. 349.
While the question of whether Fermi's experiment had produced element 93 was stalemated, two additional claims of the discovery of the element appeared, although unlike Fermi, they both claimed to have observed it in nature. The first of these claims was by Czech engineer Odolen Koblic in 1934 when he extracted a small amount of material from the wash water of heated pitchblende. He proposed the name bohemium for the element, but after being analyzed it turned out that the sample was a mixture of tungsten and vanadium.Hoffman, p. 118. The other claim, in 1938 by Romanian physicist Horia Hulubei and French chemist Yvette Cauchois, claimed to have discovered the new element via spectroscopy in minerals. They named their element sequanium, but the claim was discounted because the prevailing theory at the time was that if it existed at all, element 93 would not exist naturally. However, as neptunium does in fact occur in nature in trace amounts, as demonstrated when it was found in uranium ore in 1952, it is possible that Hulubei and Cauchois did in fact observe neptunium.
Although by 1938 some scientists, including Niels Bohr, were still reluctant to accept that Fermi had actually produced a new element, he was nevertheless awarded the Nobel Prize in Physics in November 1938 "for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". A month later, the almost totally unexpected discovery of nuclear fission by Hahn, Meitner, and Otto Frisch put an end to the possibility that Fermi had discovered element 93 because most of the unknown half-lives that had been observed by Fermi's team were rapidly identified as those of fission products.Rhodes, pp. 264–267.Rhodes, p. 346.
Perhaps the closest of all attempts to produce the missing element 93 was that conducted by the Japanese physicist Yoshio Nishina working with chemist Kenjiro Kimura in 1940, just before the outbreak of the Pacific War in 1941: they bombarded 238U with fast neutrons. However, while slow neutrons tend to induce neutron capture through a (n, γ) reaction, fast neutrons tend to induce a "knock-out" (n, 2n) reaction, where one neutron is added and two more are removed, resulting in the net loss of a neutron. Nishina and Kimura, having tested this technique on 232thorium and successfully produced the known 231Th and its long-lived beta decay daughter 231protactinium (both occurring in the natural decay chain of 235U), therefore correctly assigned the new 6.75-day half-life activity they observed to the new isotope 237U. They confirmed that this isotope was also a beta emitter and must hence decay to the unknown nuclide 23793. They attempted to isolate this nuclide by carrying it with its supposed lighter congener rhenium, but no beta or alpha decay was observed from the rhenium-containing fraction: Nishina and Kimura thus correctly speculated that the half-life of 23793, like that of 231Pa, was very long and hence its activity would be so weak as to be unmeasurable by their equipment, thus concluding the last and closest unsuccessful search for transuranic elements.
However, as more information about fission became available, the possibility that the fragments of nuclear fission could still have been present in the target became more remote. McMillan and several scientists, including Philip H. Abelson, attempted again to determine what was producing the unknown half-life. In early 1940, McMillan realized that his 1939 experiment with Segrè had failed to test the chemical reactions of the radioactive source with sufficient rigor. In a new experiment, McMillan tried subjecting the unknown substance to HF in the presence of a reducing agent, something he had not done before. This reaction resulted in the sample precipitating with the HF, an action that definitively ruled out the possibility that the unknown substance was a rare-earth metal. Shortly after this, Abelson, who had received his graduate degree from the university, visited Berkeley for a short vacation and McMillan asked the more able chemist to assist with the separation of the experiment's results. Abelson very quickly observed that whatever was producing the 2.3-day half-life did not have chemistry like any known element and was actually more similar to uranium than a rare-earth metal. This discovery finally allowed the source to be isolated and later, in 1945, led to the classification of the actinide series. As a final step, McMillan and Abelson prepared a much larger sample of bombarded uranium that had a prominent 23-minute half-life from 239U and demonstrated conclusively that the unknown 2.3-day half-life increased in strength in concert with a decrease in the 23-minute activity through the following reaction:
This proved that the unknown radioactive source originated from the decay of uranium and, coupled with the previous observation that the source was different chemically from all known elements, proved beyond all doubt that a new element had been discovered. McMillan and Abelson published their results in a paper entitled Radioactive Element 93 in the Physical Review on May 27, 1940. They did not propose a name for the element in the paper, but they soon decided on the name neptunium since Neptune is the next planet beyond Uranus in the Solar System, which uranium is named after.Rhodes, pp. 348–350.Yoshida et al., p. 700.The name neptunium was used by R.Hermann in 1877 for a chemical element which, in his opinion, could be separated from a mineral tantalite; actually, this was a misidentification. See McMillan and Abelson's success compared to Nishina and Kimura's near miss can be attributed to the favorable half-life of 239Np for radiochemical analysis and quick decay of 239U, in contrast to the slower decay of 237U and extremely long half-life of 237Np.
In 1942, Hahn and Fritz Strassmann, and independently Kurt Starke, reported the confirmation of element 93 in Berlin. Hahn's group did not pursue element 94, likely because they were discouraged by McMillan and Abelson's lack of success in isolating it. Since they had access to the stronger cyclotron at Paris at this point, Hahn's group would likely have been able to detect element 94 had they tried, albeit in tiny quantities (a few becquerels).
Neptunium's unique radioactive characteristics allowed it to be traced as it moved through various compounds in chemical reactions, at first this was the only method available to prove that its chemistry was different from other elements. As the first isotope of neptunium to be discovered has such a short half-life, McMillan and Abelson were unable to prepare a sample that was large enough to perform chemical analysis of the new element using the technology that was then available. However, after the discovery of the long-lived 237Np isotope in 1942 by Glenn Seaborg and Arthur Wahl, forming weighable amounts of neptunium became a realistic endeavor. Its half-life was initially determined to be about 3 million years (later revised to 2.144 million years), confirming the predictions of Nishina and Kimura of a very long half-life.
Early research into the element was somewhat limited because most of the nuclear physicists and chemists in the United States at the time were focused on the massive effort to research the properties of plutonium as part of the Manhattan Project. Research into the element did continue as a minor part of the project and the first bulk sample of neptunium (as neptunium dioxide) was isolated in 1944.
Much of the research into the properties of neptunium since then has been focused on understanding how to confine it as a portion of nuclear waste. Because it has isotopes with very long half-lives, it is of particular concern in the context of designing confinement facilities that can last for thousands of years. It has found some limited uses as a radioactive tracer and a precursor for various nuclear reactions to produce useful plutonium isotopes. However, most of the neptunium that is produced as a reaction byproduct in nuclear power stations is considered to be a waste product.
Heavier isotopes of neptunium decay quickly, and lighter isotopes of neptunium cannot be produced by neutron capture, so chemical separation of neptunium from cooled spent nuclear fuel gives nearly pure 237Np. The short-lived heavier isotopes 238Np and 239Np, useful as radioactive tracers, are produced through neutron irradiation of 237Np and 238U respectively, while the longer-lived lighter isotopes 235Np and 236Np are produced through irradiation of 235U with and in a cyclotron.
Artificial 237Np metal is usually isolated through a reaction of 237NpF3 with liquid barium or lithium at around 1200 °Celsius and is most often extracted from spent nuclear fuel rods in kilogram amounts as a by-product in plutonium production.
By weight, neptunium-237 discharges are about 5% as great as plutonium discharges and about 0.05% of spent nuclear fuel discharges. However, even this fraction still amounts to more than fifty tons per year globally.
Most methods that separate neptunium ions exploit the differing chemical behaviour of the differing oxidation states of neptunium (from +3 to +6 or sometimes even +7) in solution. Among the methods that are or have been used are: solvent extraction (using various , usually denticity β-diketone derivatives, organophosphorus compounds, and amine compounds), chromatography using various ion exchange or chelation resins, coprecipitation (possible matrices include LaF3, BiPO4, barium sulfate, Fe(OH)3, and MnO2), electroplating, and biotechnology methods.Yoshida et al., pp. 705–17. Currently, commercial reprocessing plants use the Purex process, involving the solvent extraction of uranium and plutonium with tributyl phosphate.Yoshida et al., p. 710.
| K(2.2.2-crypt)NpCp'3 |
| Neptunium(III) chloride, NpCl3 |
| Neptunium(IV) oxide, NpO2 |
| Neptunium(V) fluoride, NpF5 |
| Neptunium(VI) fluoride, NpF6 |
| Neptunium(VII) oxide-hydroxide, NpO2(OH)3 |
It hydrolyzes in basic solutions to form NpO2OH and .
Neptunium(III) hydroxide is quite stable in acidic solutions and in environments that lack oxygen, but it will rapidly oxidize to the IV state in the presence of air. It is not soluble in water. Np(IV) hydroxides exist mainly as the electrically neutral Np(OH)4 and its mild solubility in water is not affected at all by the pH of the solution. This suggests that the other Np(IV) hydroxide, , does not have a significant presence.
Because the Np(V) ion is very stable, it can only form a hydroxide in high acidity levels. When placed in a 0.1 M sodium perchlorate solution, it does not react significantly for a period of months, although a higher molar concentration of 3.0 M will result in it reacting to the solid hydroxide NpO2OH almost immediately. Np(VI) hydroxide is more reactive but it is still fairly stable in acidic solutions. It will form the compound NpO3· H2O in the presence of ozone under various carbon dioxide pressures. Np(VII) has not been well-studied and no neutral hydroxides have been reported. It probably exists mostly as .
The greenish-brown NpO2 is very stable over a large range of pressures and temperatures and does not undergo at low temperatures. It does show a phase transition from face-centered cubic to orthorhombic at around 33–37 GPa, although it returns to its original phase when pressure is released. It remains stable under oxygen pressures up to 2.84 MPa and temperatures up to 400 °C. Np2O5 is black-brown in color and monoclinic with a lattice size of 418×658×409 picometres. It is relatively unstable and decomposes to NpO2 and O2 at 420–695 °C. Although Np2O5 was initially subject to several studies that claimed to produce it with mutually contradictory methods, it was eventually prepared successfully by heating neptunium peroxide to 300–350 °C for 2–3 hours or by heating it under a layer of water in an ampoule at 180 °C.
Neptunium also forms a large number of oxide compounds with a wide variety of elements, although the neptunate oxides formed with and alkaline earth metals have been by far the most studied. Ternary neptunium oxides are generally formed by reacting NpO2 with the oxide of another element or by precipitating from an alkaline solution. Lithium5NpO6 has been prepared by reacting Li2O and NpO2 at 400 °C for 16 hours or by reacting Li2O2 with NpO3 · H2O at 400 °C for 16 hours in a quartz tube and flowing oxygen. Alkali neptunate compounds Potassium3NpO5, Caesium3NpO5, and Rubidium3NpO5 are all produced by a similar reaction:
The oxide compounds KNpO4, CsNpO4, and RbNpO4 are formed by reacting Np(VII) () with a compound of the alkali metal nitrate and ozone. Additional compounds have been produced by reacting NpO3 and water with solid alkali and alkaline at temperatures of 400–600 °C for 15–30 hours. Some of these include Ba3(NpO5)2, Ba2Sodium6, and Ba2LiNpO6. Also, a considerable number of hexavalent neptunium oxides are formed by reacting solid-state NpO2 with various alkali or alkaline earth oxides in an environment of flowing oxygen. Many of the resulting compounds also have an equivalent compound that substitutes uranium for neptunium. Some compounds that have been characterized include Na2Np2O7, Na4NpO5, Na6NpO6, and Na2NpO4. These can be obtained by heating different combinations of NpO2 and Na2O to various temperature thresholds and further heating will also cause these compounds to exhibit different neptunium allotropes. The lithium neptunate oxides Li6NpO6 and Li4NpO5 can be obtained with similar reactions of NpO2 and Li2O.Yoshida et al, pp. 728–730.
A large number of additional alkali and alkaline neptunium oxide compounds such as Cs4Np5O17 and Cs2Np3O10 have been characterized with various production methods. Neptunium has also been observed to form ternary oxides with many additional elements in groups 3 through 7, although these compounds are much less well studied.
NpF6 or neptunium hexafluoride is extremely volatile, as are its adjacent actinide compounds uranium hexafluoride (UF6) and plutonium hexafluoride (PuF6). This volatility has attracted a large amount of interest to the compound in an attempt to devise a simple method for extracting neptunium from spent nuclear power station fuel rods. NpF6 was first prepared in 1943 by reacting NpF3 and gaseous fluorine at very high temperatures and the first bulk quantities were obtained in 1958 by heating NpF4 and dripping pure fluorine on it in a specially prepared apparatus. Additional methods that have successfully produced neptunium hexafluoride include reacting BrF3 and BrF5 with NpF4 and by reacting several different neptunium oxide and fluoride compounds with anhydrous hydrogen fluorides.Seaborg, G. T. and Brown, H. S. (1961) US Patent No. 2,982,604.Florin, A. E. (1943) Report MUC-GTS-2165, Declassified: January 23, 1946.
Four neptunium Oxohalide compounds, NpO2F, NpOF3, NpO2F2, and NpOF4, have been reported, although none of them have been extensively studied. NpO2F2 is a pinkish solid and can be prepared by reacting NpO3 · H2O and Np2F5 with pure fluorine at around 330 °C. NpOF3 and NpOF4 can be produced by reacting neptunium oxides with anhydrous hydrogen fluoride at various temperatures. Neptunium also forms a wide variety of fluoride compounds with various elements. Some of these that have been characterized include CsNpF6, Rb2NpF7, Na3NpF8, and K3NpO2F5.
Two neptunium , Npchlorine3 and NpCl4, have been characterized. Although several attempts to obtain NpCl5 have been made, they have not been successful. NpCl3 is produced by reducing neptunium dioxide with hydrogen and carbon tetrachloride (carbon4) and NpCl4 by reacting a neptunium oxide with CCl4 at around 500 °C. Other neptunium chloride compounds have also been reported, including NpOCl2, Cs2NpCl6, Cs3NpO2Cl4, and Cs2NaNpCl6. Neptunium Npbromine3 and NpBr4 have also been produced; the latter by reacting aluminium bromide with NpO2 at 350 °C and the former in an almost identical procedure but with zinc present. The neptunium iodide Npiodine3 has also been prepared by the same method as NpBr3.Yoshida et al, pp. 736–738.Fried, S. and Davidson, N. R. (1951) US Patent No. 2,578,416.Lemire, pp. 143–155.
Neptunium selenide compounds that have been reported include Npselenium, NpSe3, Np2Se3, Np2Se5, Np3Se4, and Np3Se5. All of these have only been obtained by heating neptunium hydride and selenium metal to various temperatures in a vacuum for an extended period of time and Np2Se3 is only known to exist in the γ allotrope at relatively high temperatures. Two neptunium oxyselenide compounds are known, NpOSe and Np2O2Se, are formed with similar methods by replacing the neptunium hydride with neptunium dioxide. The known neptunium telluride compounds Nptellurium, NpTe3, Np3Te4, Np2Te3, and Np2O2Te are formed by similar procedures to the selenides and Np2O2Te is isostructural to the equivalent uranium and plutonium compounds. No neptunium−polonium compounds have been reported.
Three neptunium arsenide compounds have been prepared, Nparsenic, NpAs2, and Np3As4. The first two were first produced by heating arsenic and neptunium hydride in a vacuum-sealed tube for about a week. Later, NpAs was also made by confining neptunium metal and arsenic in a vacuum tube, separating them with a quartz membrane, and heating them to just below neptunium's melting point of 639 °C, which is slightly higher than the arsenic's sublimation point of 615 °C. Np3As4 is prepared by a similar procedure using iodine as a transporting agent. NpAs2 crystals are brownish gold and Np3As4 is black. The neptunium antimonide compound Npantimony was produced in 1971 by placing equal quantities of both elements in a vacuum tube, heating them to the melting point of antimony, and then heating it further to 1000 °C for sixteen days. This procedure also produced trace amounts of an additional antimonide compound Np3Sb4. One neptunium-bismuth compound, NpBi, has also been reported.
The neptunium Npcarbon, Np2C3, and NpC2 (tentative) have been reported, but have not characterized in detail despite the high importance and utility of actinide carbides as advanced nuclear reactor fuel. NpC is a non-stoichiometric compound, and could be better labelled as NpC x (0.82 ≤ x ≤ 0.96). It may be obtained from the reaction of neptunium hydride with graphite at 1400 °C or by heating the constituent elements together in an electric arc furnace using a tungsten electrode. It reacts with excess carbon to form pure Np2C3. NpC2 is formed from heating NpO2 in a graphite crucible at 2660–2800 °C.
It is soluble in benzene and Tetrahydrofuran, and is less sensitive to oxygen and water than plutonium(C5H5)3 and americium(C5H5)3. Other Np(IV) cyclopentadienyl compounds are known for many : they have the general formula (C5H5)3NpL, where L represents a ligand. Neptunocene, Np(C8H8)2, was synthesized in 1970 by reacting neptunium(IV) chloride with K2(C8H8). It is isomorphous to uranocene and plutonocene, and they behave chemically identically: all three compounds are insensitive to water or dilute bases but are sensitive to air, reacting quickly to form oxides, and are only slightly soluble in benzene and toluene. Other known neptunium cyclooctatetraenyl derivatives include Np(RC8H7)2 (R = ethanol, butanol) and KNp(C8H8)·2THF, which is isostructural to the corresponding plutonium compound. In addition, neptunium have been prepared, and solvated triiodide complexes of neptunium are a precursor to many organoneptunium and inorganic neptunium compounds.
Many neptunium(IV) coordination compounds have been reported, the first one being , which is isostructural with the analogous uranium(IV) coordination compound. Other Np(IV) coordination compounds are known, some involving other metals such as cobalt (·8H2O, formed at 400 K) and copper (·6H2O, formed at 600 K). Complex nitrate compounds are also known: the experimenters who produced them in 1986 and 1987 obtained single crystals by slow evaporation of the Np(IV) solution at ambient temperature in concentrated nitric acid and excess 2,2′-pyrimidine.
The coordination chemistry of neptunium(V) has been extensively researched due to the presence of cation–cation interactions in the solid state, which had been already known for actinyl ions. Some known such compounds include the neptunyl dimer ·8H2O and neptunium glycolate, both of which form green crystals.
Neptunium(VI) compounds range from the simple oxalate (which is unstable, usually becoming Np(IV)) to such complicated compounds as the green . Extensive study has been performed on compounds of the form , where M represents a monovalent cation and An is either uranium, neptunium, or plutonium.
Since 1967, when neptunium(VII) was discovered, some coordination compounds with neptunium in the +7 oxidation state have been prepared and studied. The first reported such compound was initially characterized as · nH2O in 1968, but was suggested in 1973 to actually have the formula ·2H2O based on the fact that Np(VII) occurs as in aqueous solution. This compound forms dark green prismatic crystals with maximum edge length 0.15–0.4 millimeter.
Analogously to its neighbours, uranium and plutonium, the order of the neptunium ions in terms of complex formation ability is Np4+ > ≥ Np3+ > . (The relative order of the middle two neptunium ions depends on the and solvents used.) The stability sequence for Np(IV), Np(V), and Np(VI) complexes with monovalent inorganic ligands is fluoride > > thiocyanate > nitrate > chloride > perchlorate; the order for divalent inorganic ligands is carbonate > > sulfate. These follow the strengths of the corresponding . The divalent ligands are more strongly complexing than the monovalent ones. can also form the complex ions (M = aluminium, gallium, scandium, indium, iron, chromium, rhodium) in perchloric acid solution: the strength of interaction between the two cations follows the order Fe > In > Sc > Ga > Al. The neptunyl and uranyl ions can also form a complex together.
238Pu also exists in sizable quantities in spent nuclear fuel but would have to be separated from other isotopes of plutonium.Yoshida et al., pp. 702–3. Irradiating neptunium-237 with electron beams, provoking bremsstrahlung, also produces quite pure samples of the isotope plutonium-236, useful as a tracer to determine plutonium concentration in the environment.
In September 2002, researchers at the Los Alamos National Laboratory briefly produced the first known nuclear critical mass using a significant fraction of neptunium, in combination with shells of enriched uranium (uranium-235), discovering that the critical mass of a bare sphere of neptunium-237 "ranges from kilogram weights in the high fifties to low sixties," showing that it "is about as good a bomb material as uranium-235." The United States Federal government made plans in March 2004 to move America's supply of separated neptunium to a nuclear-waste disposal site in Nevada.
Under Redox conditions, neptunium-237 is the most mobile actinide in the deep geological repository environment of the Yucca Mountain project in Nevada. This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation. Due to its long half-life, neptunium will become the major contributor of the total radiotoxicity at Yucca Mountain in 10,000 years. As it is unclear what happens to the non-reprocessed spent fuel containment in that long time span, an extraction and transmutation of neptunium after spent fuel reprocessing could help to minimize the contamination of the environment if the nuclear waste could be mobilized after several thousand years.
Finely divided neptunium metal presents a fire hazard because neptunium is pyrophoricity; small grains will ignite spontaneously in air at room temperature.
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