Energy Level Structure and Multiple 4f¹²5d¹ Emission Bands for Tm²⁺ in Halide Perovskites: Theory and Experiment

Rapid nonradiative relaxation from high-excited electronic states to lower-excited electronic states in luminescent materials leads to radiative transitions only from the lowest excited state. This behavior is observed in most luminescent materials. One notable exception is Tm²⁺, which is known to show luminescence from up to three 4f¹²5d¹ excited states. Here we report a study that explains the deviant behavior of Tm²⁺. Using ab initio wave function-based embedded-cluster calculations in Tm²⁺ -doped CsCaBr₃ and CsCaCl₃, we show that the manifold of 4f¹²5d¹ excited states shows various energy gaps for states with parallel potential energy surfaces. This strongly limits nonradiative relaxation, enabling emission from highly excited states in the case of a large energy gap to the next lower state. We also compared calculated and measured absorption and emission spectra and radiative decay times of the emitting states, which show a remarkably good agreement between theory and experiment. Based on the new insights in the Tm²⁺ excited states, we predict that luminescence from highly excited 4f^n-15d¹ states is only expected in the heavy divalent lanthanides.