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.

[6][7] When PmCl3 is heated in the presence of H2O, the pale pink colored promethium(III) oxychloride (PmOCl) is obtained.

[6][8] 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.

[9] Unlike many other radioactive nuclides, promethium-147 does not emit alpha particles that would degrade the phosphor.