valence transition

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 GPa where black, semiconducting Sm2+S2− changes to golden metallic (Sm3++e−)S2−.
Source:
PAC, 1994, 66, 577 (Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)) on page 593