Energy levels in CaWO4:Tb3+ at high pressure

Abstract

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.