A transition observed in certain rare-earth and actinide materials in which the electronic occupation of the 4f or 5f orbital changes with external conditions for example, temperature and pressure. Example: The transition at approximately \(0.65\ \rm{GPa}\) where black, semiconducting $\ce{Sm^{2+}S^{2−}}$ changes to golden metallic $\ce{(Sm^{3+} + e^{−})S^{2-}}$.