Lead Contamination of Flux-Grown Garnets and the Effect on Optical Properties

Abstract

One occasionally observes in flux-grown gallium and aluminum garnets a purple discoloration. Combined optical absorption, emission spectrographic and electron microprobe studies of crystals showing pronounced color banding indicate a direct correlation between depth of purple color and dissolved lead content. In addition to the broad visible absorption band leading to the purple color, a sharp absorption band is observed near 2.8 μ in the discolored crystals. Since the small lead impurity cannot by itself produce either of these absorptions, they must be attributed to associated charge-compensating mechanism—e.g., charged vacancies (color centers) in the one instance and OH− replacements for oxygen ions in the other. While it can be argued that the lead impurity must be partially self-compensating, the divalent state appears to predominate.