Crystal Growth and Color Centers of Alkaline-Earth Halides

Abstract

Single crystals of CaF₂, SrCl₂, BaBr₂, and BaClF have been grown. Color centers have been developed in calcium fluoride by treatment with Ca vapor at 850°C and by gamma irradiation at room temperature. It has been observed that additive colorability of the salt depends on the rate of cooling from the additive‐coloring temperature. The gamma‐irradiated samples display a host of color centers, the formation of which appears to be dependent upon the dosage of the irradiation. Below 10⁵ rad, no coloration of the crystal is achieved by the irradiation. At higher dosages, bands due to yttrium and oxygen impurities become evident, whereas at the very high dosage of 5×10⁷ rad, a multibanded spectrum with very intense infrared absorption is obtained. These bands are believed to be indicative of the formation of defect centers by high‐dosage irradiation. Unlike the bands associated with chemical impurities, these bands are relatively unstable. They are readily bleached by heat and light, the bands in the infrared being the most unstable. Color centers have also been observed in strontium chloride, barium bromide, and barium chlorofluoride. Whereas only one band has been observed for strontium chloride, more have been observed for barium bromide and barium chlorofluoride, which may presumably be due to their noncubic symmetries. Bands produced by gamma irradiation in these crystals are unstable, and all bleach readily under ordinary room conditions.