Computer simulation of Nd, Th, Sb-doped lead tungstate

In order to understand the behaviors of different impurity cations in PbWO4 (PWO), the formation energies of the possible positions of impurities Nd, Th and Sb ions in PWO crystal are calculated by the GULP code. The defect cluster models are discussed in detail by defect chemistry and the doping mechanism of Nd, Th and Sb ions is analyzed. The calculation results show that the Nd3+ and Th4+ will occupy the Pb2+ sites and form an electrical positive center near VPb2−. Those electrical positive centers can partly or wholly compensate the negative electricity of VPb2− and can restrict two holes trapped by the Vpb2−, resulting in the restraining of the 420 nm absorption band. However, Sb-doping is quite complex due to the variation of the valence states. When the doping concentration is low, the Sb ions will present 3+ state and mainly exist in the form of clusters [SbPb+–VPb2−–SbPb+]. When it is heavily doped, the Sb ions will present 5+ state and occupy the W sites near VO2+, leading to the release of VPb2− from the defect pair [VPb2−–VO2+]. The released VPb2− will trap two holes and increase the 420 nm absorption band of the crystal. The calculation results and the conclusion accord with the experimental results.