Promethium(III) chloride

Promethium(III) chloride is a chemical compound of promethium and chlorine with the formula PmCl3. It is an ionic, water soluble, crystalline salt that glows in the dark with a pale blue or green light due to promethium's intense radioactivity.

Preparation
Promethium(III) chloride is obtained from promethium(III) oxide by heating it in a stream of dry HCl at 580 °C.

Properties
Promethium(III) chloride is a purple solid with a melting point of 655 °C. It crystallizes in the hexagonal crystal system (NdCl3 type) with the lattice parameters a = 739 pm and c = 421 pm with two formula units per unit cell and thus a calculated density of 4.19 g·cm−3. When PmCl3 is heated in the presence of H2O, the pale pink colored promethium(III) oxychloride (PmOCl) is obtained.

Applications
Promethium(III) chloride (with 147Pm) has been used to generate long-lasting glow in signal lights and buttons. This application relied on the unstable nature of promethium, which emitted beta radiation (electrons) with a half-life of several years. The electrons were absorbed by a phosphor, generating visible glow. Unlike many other radioactive nuclides, promethium-147 does not emit alpha particles that would degrade the phosphor.