Single crystals were annealed in air or vacuum at temperatures of up to 910C in order to study the resultant changes in transmission characteristics. It was found that, regardless of the origin and dopant content of the crystals, annealing in air at temperatures above 600C resulted in an enhancement of the 420nm absorption band. Similar treatment in vacuum significantly decreased this band. Absorption bands at 520 and 740nm were tentatively attributed to F and F+ centers, respectively. An electron paramagnetic resonance investigation of rare-earth trace impurities in single crystals was carried out at 4.2 to 300K. The Nd3+ and Ce3+ ions were identified, and were described by an axial spin Hamiltonian with an effective spin of S = ½.

M.Nikl, J.Rosa, K.Nitsch, H.R.Asatryan, S.Baccaro, A.Cecilia, M.Montecchi, B.Borgia, I.Dafinei, M.Diemoz, P.Lecoq: Materials Science Forum, 1997, 239-241, 271-4