Neptunium(IV) oxide

Neptunium(IV) oxide, or neptunium dioxide, is a radioactive, olive green cubic crystalline solid with the formula NpO₂. It is one of two stable oxides of neptunium, the other being neptunium(V) oxide. It emits both α- and γ-particles.

Industrially, neptunium dioxide is formed by precipitation of neptunium(IV) oxalate, followed by calcination to neptunium dioxide.

Production starts with a nitric acid feed solution containing neptunium ions in various oxidation states. First, a hydrazine inhibitor is added to slow any oxidation from standing in air. Then ascorbic acid reduces the feed solution to predominantly neptunium(IV):

Addition of oxalic acid precipitates hydrated neptunium oxalate...

...which pyrolyzes when heated:

Neptunium dioxide can also be formed from precipitation of neptunium(IV) peroxide, but the process is much more sensitive.

As a byproduct of nuclear fission reactors, neptunium dioxide can be purified by fluorination, followed by reduction with excess calcium in the presence of iodine. However, the aforementioned synthesis yields a quite pure solid, with less than 0.3% mass fraction of impurities. Generally, further purification is unnecessary.

Due to neptunium's large size, neptunium dioxide has a fluorite structure, with lattice constant a=5.43 Å. Like all fluorite structure materials, it has a space group of Fm3m. Neptunium is eight-coordinate, with a cubic coordination geometry, and oxygen is four-coordinate, with a tetrahedral coordinate geometry.