A perturbed F-center, with absorption bands at 370 and 625nm, which was associated with rare-earth ions was identified in Yb-doped samples. The dislocation density was estimated to be lower than 100/cm2. Signs of Er3+ and Ho3+ were detected in the absorption and emission spectra. Hydroxyl groups were also found.

H.Yin, P.Deng, F.Gan: Journal of Applied Physics, 1998, 83[7], 3825-8