I’m currently writing a paper on modelling the preparation of sensor materials where the interesting optics comes from lanthanide dopants. The host material is a mixed metal oxide, and a typical dopant is europium, which normally exists as an oxide in a 3+ charge state. However, the dopant needs to be reduced to a 2+ charge state before being doped into the oxide. The energetics of the process therefore involves reduction followed by doping. Explaining all this clearly is a challenge, but once it’s done, I hope to post on the subject again, explaining why the valence change is needed. Watch this space!