Defect clusters of variable composition as an origin of coloration of oxide crystals under thermal treatment and irradiation

The dominant role of inclusions of tungsten oxides with variable composition due to variable tungsten valence in the sensitivity of optical absorption of ${\mathrm{PbWO}}_4$ (PWO) scintillation crystals to thermal treatment and irradiation is demonstrated. A model for processes in the inclusions, which lead to crystal coloration and recovery of initial transparency, is discussed. The deteriorating influence of inclusions containing oxides of variable-valence ions on the radiation hardness of PWO is also illustrated by studying PWO single crystals intentionally doped with niobium. To extend the model to other oxide crystals, annealing of ${\mathrm{LiNbO}}_3$ single crystals in atmospheres of poor and rich in oxygen was performed, and a reversible coloration of the crystal due to the change of niobium valence in niobium oxide inclusions is demonstrated.