Neptunium
Oxides

Three anhydrous neptunium oxides have been reported, NpO, Np O, and Np O, though some studies have stated that only the first two of these exist, suggesting that claims of Np O are actually the result of mistaken analysis of Np O. However, as the full extent of the reactions that occur between neptunium and oxygen has yet to be researched, it is not certain which of these claims is accurate. Although neptunium oxides have not been produced with neptunium in oxidation states as high as those possible with the adjacent actinide uranium, neptunium oxides are more stable at lower oxidation states. This behavior is illustrated by the fact that NpO can be produced by simply burning neptunium salts of oxyacids in air.

The greenish-brown NpO is very stable over a large range of pressures and temperatures and does not undergo phase transitions 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. Np O is black-brown in color and monoclinic with a lattice size of 418×658×409 picometres. It is relatively unstable and decomposes to NpO and O at 420–695 °C. Although Np O 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 alkali metals and alkaline earth metals have been by far the most studied. Ternary neptunium oxides are generally formed by reacting NpO with the oxide of another element or by precipitating from an alkaline solution. Li NpO has been prepared by reacting Li O and NpO at 400 °C for 16 hours or by reacting Li O with NpO · H O at 400 °C for 16 hours in a quartz tube and flowing oxygen. Alkali neptunate compounds K NpO, Cs NpO, and Rb NpO are all produced by a similar reaction:

The oxide compounds KNpO, CsNpO, and RbNpO are formed by reacting Np(VII) () with a compound of the alkali metal nitrate and ozone. Additional compounds have been produced by reacting NpO and water with solid alkali and alkaline peroxides at temperatures of 400–600 °C for 15–30 hours. Some of these include Ba (NpO), Ba Na NpO, and Ba LiNpO. Also, a considerable number of hexavalent neptunium oxides are formed by reacting solid-state NpO 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 Na Np O, Na NpO, Na NpO, and Na NpO. These can be obtained by heating different combinations of NpO and Na O to various temperature thresholds and further heating will also cause these compounds to exhibit different neptunium allotropes. The lithium neptunate oxides Li NpO and Li NpO can be obtained with similar reactions of NpO and Li O.

A large number of additional alkali and alkaline neptunium oxide compounds such as Cs Np O and Cs Np O 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.
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