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.Gmelins Handbuch der anorganischen Chemie, System No. 39, p. 61–62.
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.Weigel: Die Chemie des Promethiums, p. 588–589.Gmelins Handbuch der anorganischen Chemie, System No. 39, p. 181. When PmCl3 is heated in the presence of H2O, the pale pink colored promethium(III) oxychloride (PmOCl) is obtained.Gmelins Handbuch der anorganischen Chemie, System No. 39, p. 31.
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.
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