Photoinduced (WO4)3--La3+ center in PbWO4: Electron spin resonance and thermally stimulated luminescence study

Electron traps based on regular Pb sites perturbed by oxygen vacancies (VO) in high purity undoped PbWO4 have been revealed and studied by electron spin resonance ~ESR! and thermally stimulated luminescence ~TSL! measurements. Detailed analysis of ESR spectra parameters (g factor, hyperfine, and superhyperfine structures! along with the variation of the concentration after annealing in inert and oxygen atmospheres had shown that revealed centers are Pb1-VO complexes. The Pb1-VO centers are created in two ways: ~i! by direct trapping of the electrons from the conduction band under x-ray or ultraviolet irradiation at T560 K; ~ii! by retrapping of electrons freed from unperturbed (WO4)32 centers after irradiation at T,40 K, followed by heating at T560 K. Above T'180 K, Pb1-VO centers are thermally ionized, giving rise to a TSL glow peak due to the recombination of freed electrons with localized holes. The emission spectrum of the TSL features one band peaking at around 2.5 eV. The temperature dependence of both TSL and ESR intensities has been analyzed using a first order recombination model: the thermal ionization energy of Pb1-VO centers turns out to be 0.55 eV. The role of Pb1-VO centers in the processes of carrier trapping and recombination of nonequilibrium carriers in the PbWO4 lattice is briefly discussed