Energy levels in CaWO4:Tb3+ at high pressure

The luminescence properties of Tb3+ in CaWO4 crystals are investigated under a hydrostatic pressure of up to 200 kbar, i.e. across scheelite-to-fergusonite phase transition. It is shown that the typical blue (5D3) and green (5D4) emissions in this material are progressively quenched at room temperature as pressure is increased. This quenching is caused by a downshift of the charge transfer (or impurity trapped exciton) state that is formed between Tb3+ and nearby W6+ cations in conjunction with a pressure-induced increase of the lattice relaxation experienced by this excited state. An empirical model is introduced to calculate the evolution of the (Tb3+-W6+) charge transfer energy with pressure. Combined with the pressure dependence of the energy bandgap in CaWO4, the model allows locating the 4f levels of Tb3+ relative to the fundamental host lattice for any pressure in the range 0-200 kbar.