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

Abstract

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 4fn-15d1 states is only expected in the heavy divalent lanthanides.