Promethium(III) oxide is a compound with the formula Pm<sub>2</sub>O<sub>3</sub>. It is the most common form of promethium.
Promethium oxide exists in three major crystalline forms:
<small>*a, b and c are lattice parameters, Z is the number of formula units per unit cell, density is calculated from X-ray data.</small>
The low-temperature cubic form converts to the monoclinic structure upon heating to 750âÂÂ800 ðC, and this transition can only be reversed by melting the oxide. The transition from the monoclinic to hexagonal form occurs at 1740 ðC.