Reactions in Crystalline Lattices: Chemistry of Lower Valence States of Lanthanides

Reduction of rare‐earth (RE) ions to lower valence states in ionic crystals is described in terms of the equilibrium distribution theory for the (alkali halide):RE²⁺ and (alkaline‐earth halide):RE³⁺ systems. Thermoluminescence glow curves of radiation‐reduced CaF₂:RE³⁺ systems reveal that the reoxidation of the divalent ions apparently occurs through thermal excitation of an $f$ electron to the $d$ bands. The additive reduction of RE²⁺ to the monovalent state in alkali halides yields striking results: While Sm¹⁺ ions in KCl obtained by gamma radiation give rise to broad‐band optical spectra which were attributed to ${\text{4f}}^6{\text{6s}}^1{\text{\hspace{0.17em}→\hspace{0.17em}4f}}^5{\text{6s}}^2$ transitions, those obtained through high‐temperature additive treatment with liquid K give rise to narrow‐line spectra that are not easily interpreted. These results illustrate well the major differences between the radiation reduction and additive reduction processes.