Thermal quenching of lanthanide luminescence via charge transfer states in inorganic materials

There are various routes of luminescence quenching such as multi-phonon relaxation from excited states to lower energy states, energy migration to killer sites, and radiation less relaxation to the ground state via the crossing point in a configurational coordinate diagram. In this work, we will consider and review quenching of lanthanide luminescence by means of charge carrier transfer to the valence band or the conduction band of the host compound. We will focus on 4fⁿ-4fⁿ emission quenching due to thermally activated electron transfer from the Pr^{3+ 3}P₀ level and the Tb^{3+ 5}D₄ level to the conduction band, and due to thermally activated hole transfer from the Eu^{3+ 5}D₀ level to the valence band. In addition, we will consider the quenching of the 4fⁿ⁻¹5d-4fⁿ emission of Eu²⁺ and Ce³⁺ which often (if not always) proceeds by electron transfer to the conduction band. Since all the above quenching routes involve reduction or oxidation of lanthanides, the location of the lanthanide charge transition levels with respect to the host bands is crucial. In other words, we need to know the location of the ground and excited states in the band gap or equivalently the vacuum referred binding energies (VRBE) in the lanthanide states as can be established using the (refined) chemical shift model. A clear correlation between the temperature T₅₀ at which luminescence intensity or luminescence decay time has dropped by 50% and thermal quenching activation energies ΔE derived from VRBE schemes will be demonstrated. Since T₅₀ typically changes 400-800 K with a 1 eV change in ΔE, and since VRBE energies may contain 0.3-0.5 eV error, it will be clear that the accurate prediction of quenching temperatures from the VRBE data is not yet feasible. Nevertheless, one may derive trends and provide guidelines on how to improve the thermal stability of luminescence.