Investigating material trends and lattice relaxation effects for understanding electron transfer phenomena in rare-earth-doped optical materials

Rare-earth-doped insulators and semiconductors play an important role in a wide range of modern optical technologies. Knowledge of the relative energies of rare-earth ions’ localized electronic states and the band states of the host crystal is important for understanding the properties of these materials and for determining the potential material performance in specific applications such as lasers, phosphors, and optical signal processing. Current understanding of the systematic variations of electron binding energies in these materials is reviewed with analysis of how lattice relaxation affects the results obtained from different experimental techniques. Detailed examples are presented for rare-earth-doped YAG and LaF₃ material systems. A method for predicting the chemical shift of the 4f electrons of rare-earth impurities from the host crystal’s photoemission spectrum is also demonstrated. Furthermore, a simple model is presented that predicts host-dependent trends in the binding energies of the rare-earth ion states in materials ranging from the elemental metals to the ionic fluorides. By understanding the systematic changes in the relative energies for different states, different ions, and different host materials, insight is gained into electron transfer transitions, valence stability, and luminescence quenching that can accelerate the development of materials for optical applications.