Electrolytic Reduction of Trivalent Rare-Earth Ions in Alkaline-Earth Halides

Abstract

Trivalent dysprosium, thulium, and holmium ions in calcium fluoride, strontium chloride, and barium bromide have been successfully reduced to the divalent state by solid-state electrolysis. In contrast with similar samples reduced by gamma irradiation which are unstable with respect to light and heat, the electrolytically induced divalent rare-earth ions are optically and thermally stable. Unlike the gamma-reduced samples, recombination hole centers are believed to be absent in these samples. The majority of trivalent ions have been reduced, and the concentration of divalent rare-earth ions obtainable by the electrolytic process far exceeds that by gamma irradiation in all the samples, with or without charge-compensating monovalent cations. The completeness of the reduction, the stability of the reduced samples and, therefore, the apparent absence of recombination hole centers suggest that the reduction of the rare-earth ions follows the migration of the corresponding halide ions (which, in the case of calcium fluoride, served as charge compensators for the trivalent ions) to the anode, where they are presumably oxidized to the neutral state and escape from the crystal lattice. This mechanism is not unlike that proposed for the electrolytic coloration of alkali halides.