valence transition

https://doi.org/10.1351/goldbook.V06592
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 \(0.65\ \text{GPa}\) 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 (https://doi.org/10.1351/pac199466030577)