Abstract
Single crystals of 
, purified and containing additions of 
, 
, and equimolar quantities of 
and 
, were grown from the melt in graphite crucibles in a helium atmosphere.
Physical constants of 
were measured. Optical transmission spectra were obtained before and after x‐irradiation on slices of single crystals (about 1.5 mm thick), at 28°, −78°, − 190°C. Crystals containing 0.05–4 mole % 
, when irradiated at 28° or at −78°C, showed a radiation‐induced absorption near the fundamental absorption edge; this induced absorption is stable at 28°C. These same crystals, when irradiated at −190°C, showed another absorption which is unstable at −78°C. 
, pure, and containing low concentrations of 
, 
, or equimolar 
and 
, all showed one x‐ray induced absorption band, unstable above −10°C. 
x‐irradiated at room temperature showed a very stable absorption band.