stylistic:tone
Other inorganic

Neptunium reacts with hydrogen in a similar manner to its neighbor plutonium, forming the hydrides NpH (face-centered cubic) and NpH (hexagonal). These are isostructural with the corresponding plutonium hydrides, although unlike PuH, the lattice parameters of NpH become greater as the hydrogen content (x) increases. The hydrides require extreme care in handling as they decompose in a vacuum at 300 °C to form finely divided neptunium metal, which is pyrophoric.

Being chemically stable, neptunium phosphates have been investigated for potential use in immobilizing nuclear waste. Neptunium pyrophosphate (α-NpP O), a green solid, has been produced in the reaction between neptunium dioxide and boron phosphate at 1100 °C, though neptunium(IV) phosphate has so far remained elusive. The series of compounds NpM (PO), where M is an alkali metal (Li, Na, K, Rb, or Cs), are all known. Some neptunium sulfates have been characterized, both aqueous and solid and at various oxidation states of neptunium (IV through VI have been observed). Additionally, neptunium carbonates have been investigated to achieve a better understanding of the behavior of neptunium in geological repositories and the environment, where it may come into contact with carbonate and bicarbonate aqueous solutions and form soluble complexes.