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

The localization of electrons at W6+ sites perturbed by lanthanum in PbWO4 is studied by electron spin resonance (ESR) and thermally stimulated luminescence (TSL) measurements. The (WO4)(3-)-La3+ centers are created at the W6+ sites close to La3+ in two different ways: (i) direct trapping of electrons from the conduction band under ultraviolet or x-ray irradiation at T=60 K; (ii) retrapping of electrons freed from unperturbed (WO4)(3-) centers after irradiation at T<40 K followed by heating up to T around 60 K. Electron transfer from La3+-perturbed to unperturbed W6+ sites Stimulated by red Light illumination is also observed. The proposed mechanism of electron localization at one of four equivalent tungstate ions close to La3+ is based on the pseudo-Jahn-Teller effect, which gives rise to a rhombic distortion of (WO4)(3-) complex. At T <approximate to>95-98 K the (WO4)(3-)-La3+ centers are thermally ionized giving rise to a TSL glow peak due to the recombination of detrapped electrons with localized holes. The emission spectrum of the TSL features one band peaking at 2.8 eV. The temperature dependence of both TSL and ESR intensity is analyzed in the frame of a general order recombination model. The thermal ionization energy of (WO4)(3-)-La3+ centers has been calculated to be approximately 0.27 eV.