Neptunium trioxideNeptunium trioxide is an inorganic compound of neptunium and oxygen with the chemical formula.SynthesisNeptunium trioxide can be obtained by oxidating neptunium(V) nitrate solution in a molten eutectic mixture of potassium and lithium nitrates with ozone at 150 °C. The reaction by-products are washed out with water. Other methods are also known.Physical propertiesNeptunium trioxide is highly volatile and exists only in the hydrated form. It is insoluble in water. It forms dark brown hydrate crystals of variable composition, where x = 1–2. The hydrate crystals are of orthorhombic system.