Promethium(III) oxide

Promethium(III) oxide is a compound with the formula Pm₂O₃. It is the most common form of promethium.

Crystal structure

Promethium oxide exists in three major crystalline forms:

Form	Pearson symbol	Space group	No.	a,b,c (nm)	β(deg)	Z	Density (g/cm3)
Cubic	cI80	Ia3	206	1.099		16	6.85
Monoclinic	mS30	C2/m	12	1.422; 0.365; 0.891	100.1	6	7.48
Hexagonal	hP5	P3m1	164	0.3802; 0.3802; 0.5954		1	7.62

*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.