Photoinduced(WO4)3−−La3+center inPbWO4:Electron spin resonance and thermally stimulated luminescence study

Abstract

Electron traps based on regular Pb sites perturbed by oxygen vacancies ${(V}_{\mathrm{O}})$ in high purity undoped ${\mathrm{PbWO}}_{4}$ 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 ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ complexes. The ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers are created in two ways: (i) by direct trapping of the electrons from the conduction band under x-ray or ultraviolet irradiation at $T=60 \mathrm{K};$ (ii) by retrapping of electrons freed from unperturbed $({\mathrm{WO}}_{4}{)}^{3\ensuremath{-}}$ centers after irradiation at $Tl40 \mathrm{K},$ followed by heating at $T=60 \mathrm{K}.$ Above $T\ensuremath{\approx}180 \mathrm{K},$ ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ 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 $2.5 \mathrm{eV}.$ The temperature dependence of both TSL and ESR intensities has been analyzed using a first order recombination model: the thermal ionization energy of ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers turns out to be $0.55 \mathrm{eV}.$ The role of ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers in the processes of carrier trapping and recombination of nonequilibrium carriers in the ${\mathrm{PbWO}}_{4}$ lattice is briefly discussed.