Radiation-Induced Long-Lived Transients and Metal Particle Formation in Solid KCl-MgCl2Mixtures

Abstract

The electron paramagnetic resonance and diffuse reflectance-optical absorption spectra of room-temperature -irradiated KCl-MgCl2 binary solid salt mixtures (98:2 mol % and 2:98 mol %) and the eutectic (68:32 mol %) are reported. Additionally, powder X-ray diffraction of the pristine salts and thermal annealing studies of the irradiated salts were performed to evaluate the radiolysis product stability, annihilation, and association in metallic particles. The main long-lived transient species detected in 98:2 mol % KCl-MgCl2 salts were perturbed F-centers, that is, trapped electrons (et-) in the vicinity of Mg ions (λmax at 561 nm), and the radiolytic reduction of Mg2+ to Mg+ and Mg0. Thermal annealing promoted the diffusion of defects to yield polycations (Mgn+). On the other hand, irradiation of 2:98 mol % KCl/MgCl2 salts showed the formation of cationic and neutral Mg dimers (Mg2+ and Mg2) and trimers as well as centers with a rhombic powder pattern apparently consisting of an electron shared between three Mg2+ nuclei associated with an anion vacancy (νa+-Mg35+). Trapped electrons (et-) (F-centers) were not observed in the irradiated eutectic mixture; instead, Mg2, Mg0, and Cl3- were observed. It was observed that the higher temperature for thermal ionization of radiation-reduced Mg species decreased the extent of electron recombination reactions and the disproportionation of Cl3- compared to the pure KCl but enhanced the aggregation of Mg into larger metallic microstructures (metallic particles).