Promethium(III) oxide is a compound with the formula Pm2O3. It is the most common form of promethium.
Promethium oxide exists in three major crystalline forms:
||1.422; 0.365; 0.891
||0.3802; 0.3802; 0.5954
*a, b and c are lattice parameters, Z is the number of formula units per unit cell, density is calculated from X-ray data.
The low-temperature cubic form converts to the monoclinic structure upon heating to 750–800 °C, and this transition can only be reversed by melting the oxide. The transition from the monoclinic to hexagonal form occurs at 1740 °C.