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 lumine scence (TSL) measurements. The (WO4)(3-)-La3+ centers are created at the W6 + sites close to La3+ in two different ways: (i) direct trapping of electro ns from the conduction band under ultraviolet or x-ray irradiation at T=60 K; (ii) retrapping of electrons freed from unperturbed (WO4)(3-) centers af ter 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 Li ght illumination is also observed. The proposed mechanism of electron local ization 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 recombinati on of detrapped electrons with localized holes. The emission spectrum of th e TSL features one band peaking at 2.8 eV. The temperature dependence of bo th TSL and ESR intensity is analyzed in the frame of a general order recomb ination model. The thermal ionization energy of (WO4)(3-)-La3+ centers has been calculated to be approximately 0.27 eV.