Investigation of the quenching mechanisms of Tb3+ doped scheelites

The luminescence of terbium-doped lead tungstate (PbWO4:Tb3+) features not only emission from the 5D4 and 5D3 excited states of Tb3+, but also host and defect related broad emission bands are found. The blue host emission is attributed to the WO42− centres whereas the green emission can be ascribed to WO3 defects. Upon host excitation, Tb3+ emission is observed pointing to energy transfer mechanisms between host and dopants. The electronic structure of Tb3+ defects inside the PbWO4 host are empirically deduced from optical and luminescence spectroscopy, both in steady-state as well as in time-resolved mode, as a function of temperature and doping concentration to asses the influence of key parameters in the energy transfer processes. The luminescence originating from Tb3+ ions shows a strong dependency on both the excitation wavelength and the temperature. For instance, an intensity increase in the 75-125 K range upon excitation via the WO42− centres is found, which is absent for direct excitation. The undoped sample is characterized by a temperature-dependent energy transfer from WO42− to WO3 defect centres with an thermal energy barrier of 0.26 eV. The divergent thermal quenching profiles of the host emission for pure PbWO4 versus doped materials reveal both a direct energy transfer and a temperature dependent energy transfer process from the host towards the Tb3+ ions. The emission efficiency of the 5DJ levels is investigated as well and a thermal quenching energy of 0.51 eV (J=3) and 0.86 eV (J=4) was found.