ThO2

Thorium dioxide, chemical formula ThO2, is the oxide of thorium. It is a white to pale yellow crystalline solid that adopts the fluorite-type cubic crystal structure. In this structure, Th4+ ions occupy the cation lattice points while oxide ions occupy the tetrahedral sites, resulting in a dense oxide with a near-cubic fluorite arrangement.

ThO2 has a very high melting point (approximately 3,300 °C) and a density around 11.7 g/cm3. It is insoluble in water and relatively chemically inert, but dissolves in strong acids to form thorium salts. The compound is radioactive due to the presence of thorium-232 and other isotopes, emitting alpha particles.

Thorium dioxide occurs in nature as minerals such as thorianite (ThO2) and in accessory thorium-bearing minerals within monazite sands. Industrially it is produced by processing thorium-containing ores and by calcination or oxidation of thorium compounds to ThO2.

Applications include use as a refractory ceramic because of its high melting point and chemical stability. Historically it was employed in gas mantles (Welsbach mantles) to produce bright light. In the nuclear field, ThO2 is studied as a potential ceramic fuel form for thorium-based reactors and as a matrix for immobilizing radioactive waste.

Thorium dioxide is radioactive; the main isotope, Th-232, has a half-life of about 1.405 × 10^10 years and emits alpha radiation. Handling requires appropriate radiological controls; waste and residues are regulated; long-term stewardship is important.