Doping Mechanisms of La, Y and Excess Oxygen in PbWO₄

Abstract

The doping mechanisms of lead tungstate doped with a rare earth (Re=La or Y) and with or without excess oxygen have been clarified on the basis of dielectric spectroscopic evidence. This tungstate has emerged as an important scintillation material for a high-energy-particle detector, and its radiation hardness may be enhanced by such doping or by oxygen annealing. We studied the dielectric relaxation induced by Re impurity, which we attribute to the defect complexes [2(Re Pb 3+ ) • –(V Pb ) ′′ ]. However, the two Re differ in certain respects: La suppresses dc conductivity, yet Y increases it. This difference can be explained by our proposal that in the lattice whereas La 3+ ions all substitute for Pb 2+ , some Y 3+ occupy interstitial sites as well. We have also identified a new kind of dielectric relaxation in Re-doped crystals subjected to excessive annealing in oxygen, and attributed it to [2(Re Pb 3+ ) • –(O i 2- ) ′′ ].