Color Centers in Additively Colored Alkali Halide Crystals Containing Alkaline Earth Ions

The measurements of Pick on the absorption bands produced in additively colored crystals of KCl and NaCl containing additions of divalent ions are reviewed and an interpretation of the origin of these bands is presented. Specific models are given for the Z₁, Z₂, and Z₃ centers responsible for the absorption bands observed by Pick. These resemble models proposed by Pick, but differ in several essential points. It is concluded that the divalent ions remain atomically dispersed in KCl when the crystal is quenched to room temperature from elevated temperatures, whereas some of the divalent ions and their associated positive-ion vacancies migrate to F-centers during the quenching process in NaCl. The observations also suggest that the divalent ions are not very mobile in KCl until a temperature near 200^∘{}C is attained, although the pair consisting of a positive and a negative ion vacancy becomes significantly mobile in the vicinity of 100^∘{}C. A number of experiments are proposed which may provide a more stringent test of the models suggested.