It was found that the absorption spectrum of a Pb2+-doped crystal exhibited remarkable isothermal changes. At temperatures near to 80C, the absorption constants of most of the component bands decayed with time while the 316nm band grew. The change involved second-order kinetics, and the reaction velocities divided the bands into 2 groups. The defects which were responsible for the spectrum consisted of 2 types of defect; including I-V dipoles.

H.Asada: Physica Status Solidi B, 1994, 185[1], 87-92